water at the surface what happens to raindrops falling to surface? – some intercepted by...

Water at the Surface

• What happens to raindrops falling to surface?– Some intercepted by vegetation and evaporated• Amount of interception depends on plant structure

– Some infiltrates permeable ground– Some blocked by impermeable surfaces

Infiltration• Depends– Physcial characteristics of soil

• Coarse soils = greater inflitration rates

– Moisture content of soil• Dry soils absorb more moisture

– Vegetation cover• Dense cover prevents infiltration

– Nature of rainfall• Strong and long storms cause

run-off• Run-off flows to streams or

permeable soil or rock

Water Flow in Streams

• Flow driven by gravity• Steeper river slopes (gradients)

equal faster flow– Gradient = change in elevation

of 2 points/change in stream elevation between those two points

• Depth of river also important– Steep and deep streams flow

fastest• Rocks & trees & bends in

channel resist flow• Stream moves fastest in deepest

part (thalweg) of channel and slowest on sides and along bottom

Water Flow in Streams

Water Flow in Streams

River Discharge

• Water volume passing channel cross-section in certain amount of time

Q = V X ADischarge (Q), average velocity (V), cross-sectional area

of the flowing water (A) A = average depth of water times the width of the water

surface

Hydrographs

• Stream discharge varies over time

• Hydrograph is graph of river discharge

• Important for dam construction and flood control

Instruments for measuring stream flow

Water Beneath Surface

• 25% of all freshwater is groundwater

• 2 zones in ground may hold water– Vadose (aeration) zone– Saturated (phreatic) zone

Vadose Zone

• Water movement is by percolation– Water moves under influence of gravity– Percolation fastest in wet soil

• Water movement also by capillary action– Movement is from wetter to drier soil

• Water movement also through evaporation

Groundwater in Saturated Zone

• Zones in ground exist saturated with water

• Upper part of zone is water table

• Aquifers are fully or partially saturated zones– Porous sandstones and

limestones are good aquifers– Shales and mudstones are

usually aquicludes

Unconfined Aquifer and Perched Water Table

• Unconfined aquifers receive water from infiltration– Water moves from high to low

areas• Confined aquifers occur

between 2 aquicludes• Perched water tables occur

above main water table– Aquiclude is present in between

Unconfined Aquifer and Perched Water Table

Human Use of GroundwaterWell• Wells drilled into

water aquifer to extract water

• Water must be replaced or drawdown occurs– Cones of depression

form around wells– In coastal areas may

get saltwater intrusion

Slopes and Streams

Slope Erosion by Water

• Intense rain leads to runoff– Sheet flow and erosion

occurs– Rills and gullies form

moving water/sediment to streams

• Rainsplash erosion moves sediment downslope

• Vegetation impedes erosion

Drainage Basins

• River system consists of trunk stream and tributaries

• River system occupies drainage basin– Mississippi Basin is an

example• Drainage Divides (high

areas) separate basins

Drainage Basins & Sediment Yield

• Sediment Yield = sediment mass leaving drainage basin– Sediment yield/ area

basin = erosional effectiveness of river system

– Not all sediment removed. Alluvium deposited

Drainage Basins & Sediment Yield

Factors Influencing Sediment Yield

• Precipitation amounts• Relief• Vegetation • Lithology• Human Impac

Stream Work - Floods

• Defined as peak discharge or when stream overflows channel

• Small floods important in humid regions

• Small floods more frequent than large ones

• Recurrence interval = average time between floods of given magnitude

Stream Work – River Erosion

• Erosion by Hydraulic Action– Water breaking away and moving rock material

from valley floor and sides• Erosion by Abrasion– Erosion caused by moving sediment

• Corrosion– Minerals and rocks dissolved by water• Least important process

Stream Work - Transportation• Bedload – particles moved on

stream bottom– Large particles slide and roll– Sand grains move by saltation

• Suspended load –clays and silts suspended in water– Much suspended load leads

to muddy waters• Dissolved load– dissolved rock

material

Stream Work – River Deposition

Rivers deposit and erode sediment Flooding increases

discharge and depth, increasing stream power and sediment transport.

Receding flood decreases stream power and sediment is deposited

Increased sediment supply from basin leads to sediment deposition

Stream Work - Base Level

• Base Level = level below which a stream cannot erode– Oceans are ultimate base

levels– Lakes, dams are local

base levels– Large rivers are local base

levels for tributaries

Landscapes Shaped by Stream Erosion

Introduction• Rivers erode landscapes

—processes occur at same time– Widening and

deepening of valleys– Slope modification– Mass movements– Exposure of buried

• Stream modified landscapes more extensive than other types

Factors Affecting Patterns of Stream Erosion--Climate

• Debate continues about role of climate in landscape evolution

• Yet landscapes imprinted by past climate. – For example, glacial period• Rivers with great sediment load drained ice sheets

Factors Affecting Patterns of Stream Erosion--Lithology

• Rocks have different hardness and resistance to weathering and erosion– Quartzites very

resistant• Mount Monadnock

– Shales/mudstone not resistant

– Limestone resistant in dry climate but not in wet one

Factors Affecting Patterns of Stream Erosion--Geologic Structure

• Modified by streams to form landscape

– Hogbacks, Cuestas

– Ridges & Valleys—Appalachian Mountains

Factors Affecting Patterns of Stream Erosion--Geologic Structure

• Modified by streams to form landscape

– Domes—Black Hills

– Faults

• Produce scarps

• Uplifted blocks modified and valley s sediment –filled

Factors Affecting Patterns of Stream Erosion--Geologic Structure

• Modified by streams to form landscape– Faults

• Produce scarps • Uplifted blocks modified

and valleys filled with sediment

Drainage Patterns• Influenced by structure and rock type• High drainage density related to high erosion rates

– Badlands• Radial Drainage

– Develop mostly on volcanic cones• Annular Drainage

– Develops on domes• Trellis Drainage

– Develops on parallel folded or dipping sedimentary rocks of differing hardness• Rectangular Drainage

– Forms where joints or faults dominate• Dendritic Drainage

– Forms on extensive batholiths or flat-lying sedimentary rocks

Landforms of the Fluvial System

Alluvial Fans• Ephemeral streams

emptying from mountain valleys construct alluvial fans

• Alluvial fans are fan-shaped– Formed of alluvial

material– Debris flows in upper

alluvial fans• More common in deserts– But can form in humid

areas

Flow and Deposits on Alluvial Fans

• Flow emptying from valley spread– Velocity decreases,

sedimentation occurs– Unconfined water spreads across

fan in shallow channel

• Deposits are poorly stratified– Thick near mountain– Thin outward from mountains

• Alluvial fans coalesce to form bajadas

• Older parts of fan may develop desert varnish/pavement

Landforms of River Valleys-Meandering Rivers

• Bends in sinuous channel termed meanders– Erosion on outside of

bend– Deposition on inside of

bend—Pt bar– Grow and migrate– Cut-off meanders form

oxbow lakes

Landforms of River Valleys-Meandering Rivers

• Formation of oxbow lakes

Landforms of River Valleys - Braided Rivers

• River with multiple channels separated by sand or gravel bars

• Braiding forms by deposition of bar in mid-channel– Water flows around bar

forming two channels• High sediment load, steep

slopes, variable discharge, loose bank materials may cause braiding

Landforms of River Valleys - Floodplain

• Flat, low-lying ground bordering channel

• Frequently flooded and sediment deposited– Levees form on channel

margin

• Eroded by migrating meandering

• Growing pt. bar rebuilds floodplain

Landforms of River Valleys - Floodplain

• Widening of floodplain and devleopment of large meanders in meander belts

Landforms of River Valleys - Floodplain

• Flooding and levee development

Human Use of Floodplains

• Farming• Development• People do not adjust

well as risk grows• Flood control measures– Artificial levees, dams– Provide mostly artificial

security– Must assume floods will

recur

Terraces• Benches above modern

floodplain– Are older floodplains

• Develop through stream incision of floodplain– Lower floodplain develops– May have paired or mostly

unpaired terraces• May have aggradation before

terrace formation– Increased sediment supply

causes– Reduced sediment supply

then results in incision– Fill terraces produced

Terraces

• Aggradation and terrace development

Deltas

• Triangular shaped deposits where streams empty into oceans or lakes– Coarse sediments deposited at river mouth– Fine sediments carried farther– Distributary channels form transporting water and

sediments

• Delta form depends on:– Discharge and sediment transported– Configuration of seafloor– Strength of currents, waves, tides

Delta Types