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Geomorphological implications of the basement structure in the Krishna-

Godavari deltas, India

Article  in  Zeitschrift für Geomorphologie Supplementary Issues · March 2013

DOI: 10.1127/0372-8854/2012/0076

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Zeitschrift für Geomorphologie Vol. 57,1, 25–44, 2013 Article

published online April 2012

Geomorphological implications of the basement structure

in the Krishna-Godavari deltas, India

K. Nageswara Rao, P. Subraelu, K. Ch. V. Nagakumar, G. Demudu,

B. Hema Malini, A. S. Rajawat and Ajai

with 6 figures

Summary. A correlation of the morphology of the Krishna-Godavari twin delta region with

the tectonic framework of its basement along the east coast of India revealed that the Kolleru

Lake in the inter-delta region is situated over a basement graben known as Gudivada Sub-basin,

while the vast mudflat to the south of the lake coincides with the Bantumilli Sub-basin. The

8-km-wide beach ridge complex that separates the Kolleru and the southern mudflat largely

coincides with the Kaja-Kaikaluru Ridge (horst) that lies in between the Gudivada and Bantu-

milli Sub-basins (grabens). The anomalous landward meandering of the Vasishta and Vainateyam

distributaries close to the coastline and the three-fold widening during the past four decades of

the Nilarevu estuary in the Godavari delta are in line with the Matyapuri-Palakollu Fault. Sim-

ilarly, the 70-km-long straight segment of the delta-front coast between the Gautami and Vasish-

ta distributary mouths in the Godavari delta, which is rather anomalous for a prograding delta,

coincides with the landward boundary of the Ravva offshore sub-basin. Swerving of the Krish-

na River course throughout its deltaic reaches and the clock-wise bending of the active Krishna

delta lobe into the Nizampatnam offshore sub-basin appear to be associated with the Krishna

Cross Trend fault. Furthermore, the elongated nature of the Krishna delta and its visibly greater

seaward bulge than its bigger neighbour (Godavari delta) in spite of the fact that the latter re-

ceives higher sediment loads and water discharges, might also be due to tectonic influence. Al-

though the connection between the surface morphology and basement framework is hard to

establish for want of detailed shallow subsurface data, the landform anomalies point to the pos-

sible influence of neotectonics in shaping the morphologies of the Krishna-Godavari deltas.

Key words: Basement structure, Landform anomalies, neotectonics, Land subsidence, Krishna-

Godavari deltas

1 Introduction

The Krishna and the Godavari deltas, although built by two major rivers of the penin-

sular part of India, appear to coalesce into one large delta complex along its eastern

seaboard. Mapping of the landforms of the Krishna delta (Nageswara Rao 1985a);

the Godavari delta (Sambasiva Rao & Vaidyanadhan 1979, Rengamannar &

Pradhan 1991, Nageswara Rao et al. 2003); and the inter-delta plain in between

these two deltas (Nageswara Rao 1985b) revealed the occurrence of series of beach

ridges up to about 30–35 km inland from the present shoreline, and traces of number

of paleo distributary courses on both sides of the present river channels. The mor-

phology of these deltas is also characterized by extensive mangrove swamps, mud-

© 2012 Gebr. Borntraeger Verlagsbuchhandlung, Stuttgart, Germany www.borntraeger-cramer.de

DOI: 10.1127/0372-8854/2012/0076 0372-8854/12/0076 $ 5.00

26 K. Nageswara Rao et al.

flats, lagoons and barrier spits. A combined geomorphic map of the entire delta com-

plex indicated the continuity of the beach ridges in both the deltas through the inter-

delta region representing the former shorelines at different stages in the progradation

of these deltas during the Holocene (Nageswara Rao & Sadakata 1993). Besides

these typical delta forms, photo lineaments representing the possible neotectonic

activity have also been mapped in some of the previous studies on the Krishna-

Godavari (K-G) delta region. Based on the lineaments conjectured in the Krishna

delta through the interpretation of large scale aerial photographs, Nageswara Rao

(1988) observed the general orientation of a majority of the lineaments is in two pre-

ferred directions of ENE-WSW and NE-SW coinciding with the basement trends

indicating of neotectonic movements in the area. Similarly, Rengamannar & Prad-

han (1991) noted a lineament, which apparently guides the courses of the Vasishta

and Vainateyam distributaries in the lower part of the Godavari delta coincides with

a deep seated fault. In a recent study of the bathymetry and the magnetic field data

from the shelf zone off the Krishna-Godavari delta region, Subrahmanyam et al.

(2010), inferred possible tectonic control on the Godavari distributary courses.

Several studies indicated recent or ongoing tectonic activity influencing the delta

morphologies from different parts of the world. Vertical motion of land during the

Holocene especially in delta setting could be due to neotectonics besides isostatic

lowering and sediment compaction (Stanley 1997). The differential subsidence of

lagoons linked to the graben and half graben structures in the Albanian Adriatic

coastal region was regarded as the result of neotectonic activity during the last

2,000 years (Mathers et al. 1999). The formation and preservation of marshlands that

cover the ancient Tigris–Euphrates–Karun delta, lower Mesopotamia, were consid-

ered due to complex implications of neotectonic activities, sea-level changes and dif-

ferential sedimentation rates, besides climate change during the Holocene (Aqrawi

2001). Based on the fluvial archives, and the elevation differences in the longitudinal

profiles of the terrace deposits in Rhine-Meuse delta in central Netherlands, Cohen

et al. (2002) estimated a relative tectonic movement of 0.09–0.15 mm/yr, averaged

over the last 15,000 years. The analysis of the Holocene sea-level changes along the

northeastern coastal regions of Brazil revealed inconsistencies suggesting the role of

local tectonics among other things (Bezerra et al. 2003). The evolution of the lower

Ganges-Brahmaputra delta plain in the Holocene was influenced by regional sub-

sidence pattern in the tectonically active Bengal sedimentary basin (Allison et al.

2003). Deep subsidence along the Louisiana coast in USA was attributed to renewed

motion along a large normal fault, namely the Michoud Fault (Dokka 2006).

The Krishna-Godavari delta region overlies a complex tectonic basement setting

(Prabhakar & Zutshi 1993, Manmohan et al. 2003, Venkata Raju et al. 2003),

which might have some influence on the morphologies of these deltas. This study,

therefore, aims at correlating the tectonic framework of the basement and the land-

form anomalies in the Krishna-Godavari deltas along the east coast of India.

2 Regional setting

The Godavari and Krishna rivers originate in the eastern slopes of the Western Ghats

(close to the west coast of India) and flow eastward across the Indian peninsula for

1,465 km and 1,400 km respectively before decanting into the Bay of Bengal on the

Geomorphological implications of the basement structure 27

east coast of India. With many tributaries joining them on their way, the Godavari

and Krishna rivers drain respectively about 3.1 105 km2 and 2.5 105 km2, the

sec- ond and third largest river basins in India after the River Ganga. The drainage

basins of these two rivers together account for 17.4 % of the total geographical area

of the country (fig. 1a and 1b). Both the rivers traverse varied geological

formations (Vaithiyanadhan et al. 1988, Radhakrishna 1993) such as the

Tertiary Deccan Plateau basalts, Gondwana sedimentary rocks and Archaen

quartzites, khondalites and charnockites of the Eastern Ghats before entering into

the coastal plains. Semi- arid climate prevails over the region with temperatures

ranging between 22 °C in January and 35 °C in April. However, the average

annual rainfall in the drainage catchment basin of the Godavari River is about 1,100

mm (Selvam 2003), and that of the Krishna River is about 840 mm (Biggs et al.

2007).

After crossing the Eastern Ghats through a narrow gorge, the Godavari River

enters the east coast plains near Rajahmundry. At about seven km downstream of

Rajahmundry city (fig. 1c), the Godavari River bifurcates into two distributaries – the

Gautami and the Vasishta. Farther downstream, a third distributary, the Vainateyam

branches out of the Vasishta, while the terminal branching of Gautami forms the

fourth distributary mouth, the Nilarevu. Unlike the Godavari, which branched out

almost close to its delta apex, the Krishna River flows undivided for a distance of

about 60 km from its delta apex near Vijayawada, before a relatively small distribu-

tary branches out at Puligadda village (fig. 1c). Farther downstream, the river again

splits into three distributaries just within 15 km from the shoreline forming the main

delta lobe. Both these adjacent rivers built their deltas on the east coast India where

they join the Bay of Bengal.

The water discharges through the Krishna and Godavari rivers are highly vari-

able as they are essentially monsoon-driven river systems. The mean monthly data

on water discharges over a 58-year period between 1951 and 2008 through the

Krishna River recorded at the barrage at Vijayawada showed that almost 95 percent

of its total annual flow was during the monsoon season (June – November), while the

rest of the year accounts for only five percent of the total. Similarly the mean monthly

water discharges over a 41-year period (1968–2008) recorded at the barrage on the

Godavari River located at about seven km downstream from delta apex showed that

98 percent of the total annual discharge occurs during the six-month monsoon sea-

son, while the remaining half of the year accounts for just about two percent of the

total annual discharge (Nageswara Rao et al. 2010). The average annual flow into

the sea through Krishna River during the 58-year-period between 1951 and 2008

was 33.11 km3 with highest discharge of 99.24 km3 recorded in 1956 and the

lowest of 0.17 km3 in the drought year 2003. The average annual discharge

through the Godavari River was 86.13 km3 during the 41-year period between 1968

and 2008 with a maximum discharge of 176.81 km3 recorded in 1990 and a

minimum of 35.46 km3 in 1974 (Nageswara Rao et al. 2010).

The trends in suspended sediment loads into the Krishna and Godavari deltas

also showed large seasonal fluctuations. The monthly average loads of the suspended

sediment through the Krishna estuary were highest during the four-month peak

monsoon season accounting for 96 percent of the annual total loads with highest

loads in August during a 40-year period between 1966 and 2005. Similarly, the aver-

age monthly data on sediment loads along the Godavari River for a 37-year period

28

K. N

agesw

ara

R

ao e

t al.

Fig. 1. (a) Location of the Krishna-Godavari drainage basins in India. (b) Krishna-Godavari drainage basins enlarged with their deltas

enclosed in the box. (c) A mosaic of three Landsat MSS images of Path 153 – Row 049 from Feb. 26, 1973; Path 152 – Row 048 from

Jan 8, 1977, and Path 152 – Row 049 from June 1, 1977 in near infrared band (Band 6 in electromagnetic radiation wavelength range

of 0.7–0.8 μm) showing the Krishna – Godavari delta region. Note the elevations in meters of some discrete points read from topo-

graphic maps are overlaid on the images. The discontinued beach ridges appearing as islands in the Goguleru, a lagoon facing the sea

along the inter-delta coast is enlarged in the inset. NR: Nilarevu River. VTR: Vainateyam River.

Geomorphological implications of the basement structure 29

from 1970 to 2006 indicated that about 98 percent of the annual flux was in the four-

month period (July – October), while the rest of the eight months account for the

remaining two percent. However, there was a large difference in the absolute quanti-

ties of sediment delivered into sea through these two rivers. The annual average load

through the Krishna River was 3.98 million tons with a maximum of 16.69 million

tons in 1983 and a minimum of as low as 0.007 million tons in 2004 over a 40-year

period from 1966 to 2005. The sediment contribution of the Godavari River was

comparatively higher than its southern counterpart with a maximum annual load of

332.49 million tons recorded in 1990 and a minimum of 24.20 million tons in 1997 at

an average of about 112.74 million tons during the 37-year period between 1970 and

2006 (Nageswara Rao et al. 2010). In spite of the annual fluctuations, there was a

distinct downward trend in the sediment fluxes through both these rivers during the

respective periods analyzed. The annual average loads through the Krishna decreased

from 9.0 million tons during 1966–1969 to a mere 0.4 million tons during 2000–2005.

Similarly, the sediment flux in the Godavari showed a three-fold reduction during the

past four decades from an average annual load of 150.2 million tons during 1970–

1979 to about 57.2 million tons during the recent period of 2000–2006 (Nageswara

Rao et al. 2010).

The Krishna-Godavari twin delta region covering about 12,700 km2 is a low-

lying plain with most part of it is within 10 m above the mean sea level and gently

sloping towards the Bay of Bengal (fig. 1c). The length of the Krishna-Godavari

delta-front shoreline is 336 km. It faces a relatively low-energy marine environment

with microtidal (spring tide range is 1.5 m), and low to moderate wave conditions

considering that the significant wave height is 2 m only (Nageswara Rao et al.

2008). However, the region is prone to severe cyclonic activity accompanied by

stronger waves and occasional storm surges which often reach several kilometres

inland in this extremely low-lying region sloping seaward at an average gradient of

just about 0.011 %. The region is densely populated with 9.26 million people at

729 persons per km2, which is more than double the country’s average of 325 per km2

(as per the 2001 census).

3 Materials and methods

The subaerial morphology of the Krishna-Godavari (K-G) delta region has been

mapped using remote sensing images of both Landsat MSS (multispectral scanner)

from the 1973–1977 at 80 m spatial resolution and Indian Remote Sensing Satellite

(IRS) P6 – LISS III (Linear Imaging Self-scanning System – III) images from 2008

at 23.5 m spatial resolution. The Landsat images from 1970s are found more useful

for demarcating most of the relict landforms, especially the beach ridges, which are

not clearly discernible from the recent imagery (despite its relatively higher spatial

resolution) due to intense land use activity especially the aquaculture that has to a

greater extent transformed the landscape of these deltas, of late, defacing most of the

relict geomorphic features. However, the recent satellite imagery (IRS P6 LISS III

Path 102 – Row 062 dated March 3, 2008 covering the Krishna delta, and Path 103 –

Row 061 dated March 8, 2008 covering the Godavari delta including the inter delta

zone) are used to incorporate in the map the present shoreline configuration of the

deltas. The morphological features of the entire K-G delta area, including the inter-

30 K. Nageswara Rao et al.

delta zone were traced from both Landsat MSS data and IRS LISS III data in digital

format through onscreen digitization. Substantiated by the field verifications, the

final landform map thus prepared not only shows the relict landforms but also incor-

porates the recent shoreline features along the delta front coast.

The basement tectonic map of the Krishna-Godavari (K-G) sedimentary basin

compiled from Manmohan et al. (2003), Venkata Raju et al. (2003), Bastia et al.

(2006) and Lal et al. (2009) is overlaid on the landform map using the geographic

information system (GIS) techniques to correlate the basement configuration and

the delta morphology. Similarly, changes along certain sections of the distributary

channels of the Godavari during the recent decades have been brought out through a

comparison of Corona satellite photographs from 1965 and IRS LISS III images from

2008 substantiated by field evidences.

4 Basement structure

The Krishna-Godavari (K-G) delta complex occupies a pericratonic rift basin (Prab-

hakar & Zutshi 1993), located in the central part of the eastern passive continental

margin of India (Rao 2001, Gupta 2006), which was formed due to the downwarp-

ing of the eastern part of the Indian shield subsequent to the breaking up of the

Gondwanaland (Murthy et al. 1995). The K-G delta sedimentary basin extends over

an area of about 45,000 km2 from its landward limit up to about 200 m water depth

offshore. The subaerial part of the K-G sedimentary basin covers about 20,000 km2

(Govindan 2004) from north of Kakinada in the Godavari delta up to south of the

Krishna delta (inset in fig. 2). The basin which was initially formed in the Late Juras-

sic as a major rift graben is bound by two major faults – the Basin Margin Fault along

the landward (northwest) side, and the Matyapuri-Palakollu (MP) Fault, which

marks the Eocene shelf edge (Raghavendra Rao 1991) on its seaward margin with

a landward tilt along the latter (Manmohan et al. 2003). Recurrence of tectonics in

the Late Cretaceous led to en echelon faults giving rise to second order horst-graben

structures within the K-G sedimentary basin (fig. 2). These basement highs and lows

run, more or less, parallel to the coast. There are two cross-trend faults, namely

Chintalapudi Cross Trend and Krishna Cross Trend, which divide the overall K-G

sedimentary basin into three major blocks. The eastern block (east of Chintalapudi

Cross Trend), which is more or less occupied by the Godavari delta, exhibit well-

defined horst-graben structures such as Tanuku Ridge (horst), Mandapeta Sub-basin

(graben), Draksharama Ridge, Narsapur Sub-basin, and Ravva Sub-basin, in that

order from the Basin Margin Fault towards the sea. In fact, the Ravva Sub-basin is in

the present offshore zone fringing the Godavari delta front coast. The central block

(in between the two cross trend faults) is divided into Gudivada Sub-basin, Kaza-

Kaikalur Ridge and Bantumilli Sub-basin. The western block (west of the Krishna

Cross Trend) is underlain by the Bapatla Ridge in its upper part and the Nizampat-

nam Sub-basin in its seaward part.

The sediments that filled the K-G sedimentary basin range from the Cretaceous

to the Recent (Biswas & Agarwal 1999) and vary in thickness from about 2.0 to

2.5 km over the basement highs (Manmohan et al. 2003) to as much as 5.0 to 7.0 km

over the graben structures containing at least eight hiatuses (Govindan 2004).

Geomorphological implications of the basement structure 31

Fig. 2. Basement tectonic framework of the Krishna-Godavari sedimentary basin (compiled

from Manmohan et al. 2003, Venkata Raju et al. 2003, Bastia et al. 2006 and Lal et al. 2009).

Inset map in the top left corner shows the K-G sedimentary basin (both on-land and offshore

parts of it) in grey colour shade, while the area enclosed in the box within this basin is the K-G

delta area considered in this study.

5 Delta morphology

Morphologically, the entire K-G delta plain appears as a single depositional unit.

However, the two deltas are often separated, in a number of earlier studies by tak-

ing their respective limits along the lateral-most abandoned distributary courses

identifiable from the aerial photographs and satellite imagery on both sides of the

present courses of the respective rivers (e. g. Sambasiva Rao & Vaidyanadhan 1979,

Rengamannar & Pradhan 1991, Nageswara Rao et al. 2003, on Godavari delta,

and Babu 1975, Nageswara Rao 1985; on Krishna delta). Thus, out of the total

12,700 km2 area of the Holocene Krishna-Godavari twin delta complex, the Goda-

vari delta component is approximately 5,200 km2 and that of the Krishna is about

4,800 km2. The rest of the 2,700 km2 area in between these two deltas is known as

the inter-delta plain which has also been built along with the two deltas on either

side of it, and is characterized by series of lagoons like the Kolleru Lake (a former

lagoon turned into a freshwater lake) and Goguleru (present lagoon) separated by

32 K. Nageswara Rao et al.

several sets of beach ridges (Nageswara Rao 1985b). Both the delta plains exhibit

two broad units – the fluvial plain in the upper part of the delta and the strand plain

in the lower part. The former is a gently rolling, river-built plain sloping towards

the coast characterized by landforms such as abandoned river courses and natural

levees. In contrast, the lower strand plain, including the inter-delta plain, exhibits

features like beach ridges, mudflats, mangrove swamps, lagoons, and spits reflecting

the marine influence. The sandy beach ridges in the strand plain represent the for-

mer shoreline positions in these prograding deltas. The innermost beach ridge that

lies even up to 35 km inland from the present shoreline marks the maximum

Holocene transgression limit, which was tentatively dated to be around 6 ka based

on about ten C-14 dates from this region (Nageswara Rao et al. 2005). The delta

area upstream of this maximum Holocene transgression limit (shown as a thick stip-

pled line in fig. 3) is considered as the fluvial plain as it is essentially a floodplain of

either the present or abandoned distributary courses within the delta. The lower part

of the delta seaward of this limit is the strand plain. Although most part of the sed-

iments in the delta is contributed by the river, the material is reworked and deposited

by the marine processes along the shore leading to the progradation of the delta.

Therefore, the strand plain that constitutes the lower part of the delta is essentially

a marine-built plain. However, the discontinuous nature of the beach ridges at many

locations observed in the strand plain of the K-G delta region is mainly due to

subsequent fluvial deposits that overlap the strand plain as the rivers shifted their

distributary courses and reached the sea in different directions through time in the

process of delta evolution. The orientation of the beach ridges in the strand plain is

not always parallel to the present shoreline. At number of locations, the beach ridges

are oriented almost oblique to the present shoreline, such as to the northeast of

Nizampatnam in the western part of the Krishna delta; to the northeast of Machili-

patnam in the eastern side of the Krishna delta; and to the south of Vilasa in the

central part of the Godavari delta. This difference in orientation of beach ridges indi-

cates a major change in the configuration of the coastline in the past either due to a

shift in the location of distributary mouth and/or sea level change that commonly

associate with the delta growth process. Based on the disposition of the beach ridges

and the apparent location of the paleo distributary mouths, number of attempts were

made earlier towards reconstructing the different stages in the growth of the K-G

deltas during the past 6 ka as a consequence of the interaction of fluvial and marine

forces (e. g. Sambasiva Rao & Vaidyanadhan 1979, Nageswara Rao 1985a,

Nageswara Rao & Sadakata 1993, Nageswara Rao et al. 2003, 2005).

In addition to the fluvial and marine forces, the tectonic stability of the receiv-

ing basin also influences the delta morphology. It is generally believed that the geo-

morphological styles of the coastal sedimentary basins along the eastern seaboard of

India are shaped by the Quaternary and Recent movements that represent the post-

collision adjustments of the Indian plate (Biswas & Agarwal 1999). The impact of

recent tectonics on the landforms along the east coast of India is reported in several

studies. Agarwal & Mitra (1991) noted the neotectonic activities, reflected in terms

of four sets of lineaments in the Ganga delta. Similarly, Babu (1991) observed that

reactivation from time to time along the structural elements of the Precambrian base-

ment has influenced the growth of the Cauvery delta in the southern part of the east

coast of India even during the late Quaternary. According to Vaz & Banerjee (1997)

Geomorphological implications of the basement structure 33

seismogenic movements as recently as in the 19th century were responsible for dis-

placement of sediment sequences in the Pulicat Lake, a large (600 km2) lagoon along

the east coast of India. Farther southward along the east coast of India, in the state of

Tamil Nadu the geomorphologic features like anomalous bending of the Coleroon

River from NE to ESE direction, and the rapid changes in the Vedaranyam backwa-

ter that fringe the Cauvery delta, are attributed to ongoing tectonic activity (Rama-

samy & Ramesh 1999). Based on a number of riverine, coastal, and hydrological

anomalies examined, Ramasamy (2006) inferred that southern India is tectonically

very active.

Similar inferences on the tectonic influence on landforms have also been made

on the Krishna-Godavari delta region. In a comprehensive geophysical analysis for

the reconstruction of the stratigraphy of the Krishna-Godavari sedimentary basin,

Manmohan et al. (2003) interpreted continued subsidence of the Tertiary and

younger sediments due to reactivation along the basement faults. Shallow seismic

studies in the continental shelf off the Krishna-Godavari (K-G) delta region revealed

faulted structures representing the Pleistocene events (Rao 1991). As mentioned

already, several studies reported the possible tectonic activity being responsible for

some anomalous landforms from K-G deltas (e. g. Rengamannar & Pradhan 1991).

Therefore, a comparative study of the basement tectonic framework and the mor-

phology of the K-G delta region might reveal the tectonic impact on the landform

pattern.

6 Basement structure and subaerial morphology

The correlation between the basement structure and the surface morphology of the

K-G delta region is visible when the map showing the tectonic framework of the

K-G sedimentary basin (fig. 2) and that of the subaerial morphology of the K-G delta

region (fig. 3) are overlaid using GIS software. The detailed description of the indi-

vidual features is given in separate sections on the inter-delta, the Godavari delta and

the Krishna delta zones that comprise most of the subaerial part of the K-G sedimen-

tary basin.

6.1 Inter-delta zone

The low-lying coastal plain between the Krishna and Godavari deltas, which is

designated as inter-delta zone is characterized by the presence of alternating lakes/

lagoons and series of beach ridges. The most prominent feature of this zone is the

Kolleru Lake, a large freshwater body spread over 245 km2 and surrounded by much

larger lake plain which is apparently the emerged part of the lake. The lake is located

more than 35 km inland from the shoreline. A series of curvilinear beach ridges

fringes the seaward margin of the Kolleru Lake. Apparently this lake was formed ini-

tially as a coastal lagoon by submergence of low-lying land when the shoreline was

along the maximum Holocene transgression limit (Nageswara Rao 1985b), which

was tentatively dated to be around 6.3–7.3 ka (Sadakata et al. 1998, Banerjee 2000).

The Kolleru Lake has turned into a freshwater body due to diminished tidal influ-

ence, as the shoreline advanced by the progradation of the Krishna and Godavari

deltas on both sides, and also due to freshwater inputs from several rivulets, which

34 K. Nageswara Rao et al.

Fig. 3. Geomorphic features in the Krishna-Godavari delta region. Thick stippled line marks

the Holocene maximum transgression limit, the landward of it is the fluvial plain and the sea-

ward of it is the strand plain. Broken lines on both sides of the two river courses represent

the lateral extent of the Krishna and Godavari deltas, while the area in between these two deltas

is the inter-delta area.

decant into the lake. Apart from the Kolleru Lake, there have been at least two more

lagoons formed in the inter-delta zone of which Goguleru fringing the present shore-

line is the active one. In between the Kolleru Lake (past coastal lagoon) and Goguleru

(active lagoon), there is an extensive mudflat, which was thought to be a ‘filled-in’

lagoon (Nageswara Rao 1985b). The three lagoons are interspersed by two sets of

beach ridges.

The basement structure encompassing the entire inter-delta zone is more or less

a basin bound by the two shore-normal Chintalapudi and Krishna Cross Trend faults

of the basement. However, there is a narrow Kaza-Kaikalur Ridge that exists in the

basement here which separates the basin into two, namely the Gudivada and Bantu-

milli sub-basins. The Kolleru Lake is situated in the Gudivada Sub-basin, while the

vast mudflat (to the south of the Kolleru Lake) and the Goguleru (the present lagoon)

further seaward coincide with the Bantumilli Sub-basin. The 8-km-wide set of the

curvilinear beach ridges between the Gudivada-, and Bantumilli sub-basins, on the

other hand, overlies the Kaja-Kaikalur Ridge (fig. 4). The continued existence of

Kolleru Lake even after it had fallen 35 km inland might be due to its location in the

Geomorphological implications of the basement structure 35

tectonically floundered Gudivada Sub-basin. In a recent study based on bathymetry

and magnetic anomalies, Subrahmanyam et al. (2010), inferred two parallel faults in

the continental margin off the Krishna-Godavari delta and suggested that the con-

cavity of the inter-delta shoreline is due to tectonic control. Apparently, these two

offshore faults mapped by Subrahmanyam et al. (2010) are the offshore extensions

of the Chintalapudi and Krishna Cross Trend faults of the basement.

The present lagoon, viz. the Goguleru spread over an area of 48 km2 along the

inter delta shoreline, appears more like a linear body occupying low-lying swales in

between beach ridges in the area (inset in fig. 1c). There are a number of sandy islands

within the lagoon. The linearity and disposition of these islands indicate that they are

the detached portions of the beach ridges that lie to the NW as well as SE of the

lagoon. Apparently, tectonic subsidence of the area overlying the Bantumilli Sub-

basin leading to the formation of the Goguleru, as the low-lying portions of the beach

ridges and swales were filled by the invading sea, while the relatively higher parts of

the ridges stand out as islands in the lagoon.

Fig. 4. Correlation of the Holocene delta morphology (interpreted from the remote sensing

satellite imagery), and the basement tectonic framework of the Krishna-Godavari sedimentary

basin (compiled from Manmohan et al. 2003, Venkata Raju et al. 2003, Bastia et al. 2006 and

Lal et al. 2009).

36 K. Nageswara Rao et al.

6.2 Godavari delta

The most prominent feature in the Godavari delta that point to the tectonic influence

is the anomalous meandering of the distributary courses. The Vasishta distributary

course turns right after the Vainateyam branches out of it, and flows almost parallel

to the coast before it swerves rather unusually landward and then takes a U-turn

again towards the coast. A similar landward meandering is noticeable in the course

of the Vainateyam as well. The courses of both these distributaries especially in their

extreme lower reaches in such a close proximity to their mouths are rather unusual.

Fig. 5. IRS P6 LISS III image of Path 103 – Row 061 from March 8, 2008 in near infrared

band (Band 3 in electromagnetic radiation wavelength range of 0.77–0.86 μm), showing the

four distributary mouths of the Godavari River. Inset photos show pronounced coastal ero-

sion in the area: (a) breached beach ridge to the east of the Vasishta mouth; (b) abandoned oil

well which was initially 600 m inland is now in the intertidal zone; and (c) uprooted casuarina

trees to the east of Vainateyam mouth. The Nilarevu mouth area, enclosed in the box (in the

top right corner), is enlarged in figure 6.

Geomorphological implications of the basement structure 37

These anomalous sections of both the distributaries coincide with the line of Matya-

puri-Palakollu (MP) Fault in the basement (fig. 5). Since, the K-G sedimentary basin

had tilted landward along the MP Fault (Manmohan et al. 2003), the landward of

the MP Fault is a graben, which is designated as Narsapur Sub-basin. Nageswara

Rao et al. (2004) noticed that the boreholes through which natural gas was emanated

are aligned with the neotectonically active MP Fault and some other sympathetic

faults parallel to it that are also manifested by beach ridges and at places streamlined

by backwaters in the lower Godavari delta plain. Landward channel migration is

evident even at present from the sustained erosion along the undercut slope at the

U-turn sections of both the Vasishta and Vainateyam distributary courses. Many

uprooted coconut plantations and the stone-cladding of the eroding banks by the

local authorities are noticeable in the field at these locations.

Similarly, the recent changes at the Nilarevu distributary river mouth which

coincides with the eastern end of the MP Fault also seem unusual. A comparison of

the Corona satellite photograph from 1965 and the IRS LISS III image from 2008

Fig. 6. Temporal changes in the width of

the Nilarevu estuary. (a) Corona satellite

photograph from October 7, 1965; and

(b) IRS P6 LISS III image from March 8,

2008 in near infrared band (Band 3 in elec-

tromagnetic radiation range of 0.77–

0.86 μm). Overlaying of the two images

revealed the widening of the Nilarevu

course (c) almost three times by erosion (or

drowning?) between 1965 and 2008.

38 K. Nageswara Rao et al.

reveals a three-fold widening of the Nilarevu course in four decades from 700 m in

1965 to 2,400 m by 2008 over a length of 12 km in its lower reaches up to its mouth

(fig. 6). The very fact that such a widening of the estuary occurred only in the case

of Nilarevu distributary, while the estuaries of the other three distributaries of the

Godavari River have not shown any marked change in their width, rules out the

possibility of drowning of estuaries by relative sea level rise in the region as a whole.

Most likely, tectonic subsidence along the MP Fault is responsible for the drowning

of the Nilarevu estuary, which coincides with the Fault.

Another significant feature of the Godavari delta is the remarkably straight

segment of its delta-front coastline over a length of 70 km between the mouths of

the Gautami and the Vasishta distributaries (fig. 5). This is again anomalous for a pro-

grading delta like that of the Godavari. This coastal segment in the central part of the

delta coincides with the landward boundary of the Ravva offshore Sub-basin of the

basement (see fig. 2). Based on a comparison of multi-date satellite imagery, Nages-

wara Rao et al. (2005) noted pronounced erosion and shoreline retreat by about

500 m between 1965 and 2001. Subsequent field observations during 2007–2009

revealed continued erosion in this part (fig. 5a, b and c). Sustained erosion of the delta

front coast even at the distributary mouth regions in this part of the Godavari delta

on decadal scale might be the result of decreased sediment supply as a consequence

of upstream damming (Nageswara Rao et al. 2010). It is equally interesting to note

that the eroded segment of the Godavari delta front coast coincides with the land-

ward edge of the Ravva Sub-basin which might also be a factor in such pronounced

coastal erosion along this 70-km-long delta front coast.

6.3 Krishna delta

As already mentioned, there are two cross trend faults in the basement of the K-G

sedimentary Basin, the Krishna Cross Trend, and the Chintalapudi Cross Trend. The

former is located in the Krishna delta region. It runs in a NW-SE direction coincid-

ing almost with the entire length of the Krishna River course in its deltaic reaches.

The Krishna River course from Vijayawada at the delta apex up to the shoreline over

a 95-km stretch progressively swerves clock-wise to its right apparently into the

downthrown side of the Krishna Cross Trend fault (fig. 4).

Further, the Krishna delta shows distinct bulge into the sea relative to the gen-

eral shoreline trend on both sides of it. The bulge of the Krishna delta also skewed

markedly towards its right side into the Nizampatnam Bay. This type of asymmetric

delta bulge is probably due to the influence of the Krishna Cross Trend fault as the

delta tends to grow more into the downthrown side of this fault.

7 Discussion

The comparison of the Krishna-Godavari (K-G) delta morphology with its basement

tectonic framework revealed significant correlations between the tectonic elements

in the basement structure and the overlying surficial landforms. The continued exis-

tence of Kolleru Lake strongly suggests tectonic influence. Usually, formation of

lagoons and their emergence is common process associated with delta growth. In fact

some of the extensive mudflats dotted with broken beach ridges that occur in the K-G

Geomorphological implications of the basement structure 39

delta region could represent former lagoons formed by submergence of land by

subsidence analogous to the present lagoon, viz., Goguleru in the inter delta zone.

However, Kolleru Lake which appears to have formed more than 6,000 years BP

(Sadakata et al. 1998, Banerjee 2000) still persists in spite of its present location at

about 35 km inland from the shoreline. The fact that the bed level of this lake in some

parts is at or below sea level as a result of which seawater enters the lake during high

tide through a 64-km-long intricately meandering tidal channel (locally called

‘Upputeru’ – a salt stream) suggests that the area occupied by the lake is perhaps sub-

siding. A borehole drilled along the southern margin of the Kolleru Lake revealed

that the lake bed sediments occur up to about 12 m depth (Malleswara Rao &

Sekhar 2003). Piling up of such thick lake deposits might be possible due to synde-

positional subsidence. The topography in and around Kolleru Lake clearly shows

that the lake area is a depression. As can be noted from the spot heights and bench-

marks shown in figure 1c, the Kolleru Lake and its adjacent areas are within 2 m

elevation surrounded by higher elevations in all directions ranging from ~ 5 to 10 m.

The lake area including the vast lake plain shows a general NE-SW linear trend, which

largely coincides with the long axis of the Gudivada Sub-basin of the basement.

Another significant feature of the K-G delta region is the distinct difference in

the morphometry of both the deltas. Comparatively, the Krishna delta is more elon-

gated while the Godavari delta is wider. The long axis of the Krishna delta, when

measured as a straight line distance from the delta apex at Vijayawada up to the most

seaward part of the coastline is 95 km, while that of the Godavari from Rajahmundry

is 75 km. But the length of the Krishna delta-front coastline is only 140 km against

the 170-km-long Godavari delta front coast. Therefore the dimensionless value of

length by width for the Krishna delta is 0.67, which is greater than 0.44 for the

Godavari delta. The variation in the morphologies of these two deltas has similarity

to the variations in the basement structure. The horst-graben structures that are more

pronounced across the Godavari delta plain might have favoured widening of the

Godavari delta as its sediments tend to move parallel to the grabens. On the other

hand, the Krishna delta has a greater long axis which might be due to the influence of

the Krishna Cross Trend fault that guides the river course rapidly seaward. In the case

of the Godavari delta, however, the northeastward plunge of the horst-graben struc-

ture in the basement (Manmohan et al. 2003) tends to influence the sedimentation

mostly coast parallel. Therefore, the influence of the Chintalapudi Cross Trend fault

is not visible on the Godavari course.

Even on a decadal scale, the Godavari delta showed lower net accretion rate than

the Krishna delta, in spite of the higher quantities of 112.74 million tons per year

of suspended sediment delivered into the sea through the Godavari River, when

compared to the 3.98 million tons per year that the Krishna River delivers. During a

35-year pre-dam period between 1930 and 1965, the net accretion in the Godavari

delta was at an average rate of 0.30 km2 per year as against 1.15 km2 per year in

the Krishna delta (Nageswara Rao et al. 2010). Similarly, during a 43-year

period between 1965 and 2008 which witnessed construction of number of large dams

across these rivers, the net erosion rate along the Godavari delta coast was higher

at an average of 0.87 km2 per year as against 0.82 km2 per year along the

Krishna delta (Nageswara Rao et al. 2010). This unusual phenomenon of

disproportionate growth of the Godavari delta when compared to the neighbouring

Krishna delta in spite of

40 K. Nageswara Rao et al.

the fact that the Godavari River discharges more sediment loads than the Krishna

River rather exemplifies the impact of the land subsidence during Holocene and pos-

sible ongoing tectonic activity along the Godavari delta probably at a comparatively

higher rate than in its southern counterpart.

The data on water discharges into the sea through both these rivers also leads to

a similar inference. The discharge data recorded during a four-decade period (1968–

2008) indicated that the average annual outflow into the sea through the Godavari

River was about 86.13 km3 with a maximum discharge of 176.81 km3 during 1990 and

a minimum of 35.46 km3 in 1974. On the contrary, the discharges through the Krishna

River during the corresponding period was only about 24.03 km3 with a maximum

of 71.66 km3 in 1975 and a minimum of 0.17 km3 in 2003. Unlike in the case of

the Krishna River, however, the annual discharges through the Godavari River do

not show any significant decreasing trend even at present (Nageswara Rao et al.

2010). Despite higher water discharges and sediment loads, the Godavari delta

showed more of erosion and less seaward bulge when compared to the Krishna

delta may be due to tectonic subsidence.

The latest seismic zoning of India by the Bureau of Indian Standards IS 1893 –

Part 1: 2002, (http://www.bis.org.in/other/quake.htm) categorized the K-G sedimen-

tary basin region under Zone III with moderate seismic intensity. The long-term seis-

mic record (http: // www.asc-india.org/seismi/seis-andhra-pradesh.htm) shows that a

number of earthquakes occurred both on land as well as in the offshore part of the

Krishna-Godavari delta region. For instance, three earthquakes of maximum ob-

served intensities (MOI) of V to VI occurred in quick succession during July – Au-

gust 1859 near Guntur-Tenali area in the western part of the Krishna delta region.

In Godavari delta region, an earthquake of MOI of V occurred in Kakinada area on

December 19, 1869 and another one of 5.5 surface wave magnitude (Ms) occurred

on October 12, 1954 in the Bay of Bengal about 197 km ESE off Kakinada, besides

the one of 5.2 body wave magnitude (Mb) that occurred on July 5, 1974 in the sea

with its epicenter 167 km SE of Nizampatnam off the Krishna delta. It may be

noted that although different scales are used in the source data to refer to the earth-

quake magnitudes, all the scales mentioned more or less indicate similar intensities.

Seismic surveys in the continental margin off the K-G delta region revealed evi-

dences of faulting in the subsurface layers with about 5–20 m vertical displacement

(Ramana et al. 2009) even in the late Pleistocene sediments (Rao 1991). Ongole area

to the immediate south of the subaerial Krishna delta, which is considered as a part

of the overall K-G sedimentary basin (Murthy et al. 1995), also experienced inter-

mittent seismic activity including the three known earthquakes of moderate magni-

tude of ~ 5–5.8 (Reddy & Chandrakala 2004). The recurring seismicity of this

area is attributed to the periodical reactivation of faults (Reddy & Chandrakala

2004). In a similar Zone III seismic setting of the lower reaches of the Narmada

River along the west coast of India, the occurrence of uplifted fluvial terraces are

reported to represent tectonic activity during the historical times (Raj & Yadava

2009). It is reasonable, therefore, to assume that the neotectonic activity perhaps

along the lines of the horst-graben structures and the basin bounding as well as

cross-trend faults in the basement has had a role in shaping the morphologies of the

K-G deltas along the east coast of India, although there are no conclusive evidences

yet in this direction.

Geomorphological implications of the basement structure 41

8 Conclusions

The morphology of the Krishna-Godavari delta region apparently bears the imprints

of the neotectonic activity more or less coinciding with the structural framework of

the basement at least in some parts. The continued existence of the Kolleru Lake, a

former lagoon, which coincides with the basement graben structure; anomalous

meandering of the Vasishta and Vainateyam distributaries, besides the recent three-

fold widening of the estuary of the Nilarevu distributary that coincide the MP Fault

in the Godavari delta; anomalous straightening and pronounced erosion of the

Godavari delta-front coast that fringe the Ravva Sub-basin; clock-wise swerving of

the Krishna River besides the elongated nature of the Krishna delta when compared

to the neighbouring Godavari delta and its asymmetric growth more to its right may

be indicative of morphotectonic processes and ongoing tectonic activity in the region.

However, detailed geophysical investigations for profiling the subsurface conditions

are necessary to establish the role of neotectonics and its connection, if any to the

basement.

Acknowledgements

We thank Dr R. R. Navalgund, Director, and Dr J. S. Parihar, Deputy Director, Space Applica-

tions Centre, Ahmedabad for encouragement. Financial support from Department of Space,

Government of India, through a research project on the coastal vulnerability of AP coast,

under Disaster Management Support Programme (DMSP) is gratefully acknowledged. We are

grateful to Prof. Dr. K.-H. Pfeffer, Chief Editor, Zeitschrift für Geomorphologie, for valuable

suggestions and encouragement which led to significant improvement of the manuscript.

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Manuscript received: September 2011; accepted: December 2011.

Addresses of the authors: K. Nageswara Rao, P. Subraelu, K. Ch. V. Nagakumar, Department

of Geo-Engineering, Andhra University, Visakhapatnam 530003, India – G. Demudu, B. Hema

Malini, Department of Geography, Andhra University, Visakhapatnam 530003, India –

A. S. Rajawat, Ajai, Space Applications Centre, Ahmedabad 380015, India.

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