Adam C. Simon 

Ph.D., University of Maryland, 2003

 Research Associate

Department of GeologyUniversity of MarylandCollege Park, MD 20742

p: 301 405 0235f: 301 314 9661

e-mail: asimon@geol.umd.edu

Stream Flow

Some of the consequences of natural stream flow present engineering and social challenges with which

we grapple year after year, and have through civilization’s history.

The flow of fresh water in channels on the Earth’s surface has been essential to the development of topography and

most ecosystems.

Global Water Budget

Although water in streams

and lakes is not significant

in terms of the global water

supply, it is a significant

component of the fresh

water budget.

Flowing surface water is

critical in erosion and

dispersal of sediment.

p.254

Water Moving Sediment on MarsWater Moving Sediment on Mars

High resolution images of the Martian surface further demonstrate

that running water was once present on the red planet’s surface.

The Hydrologic

Cycle

Development of Stream Channels

Once moving water

forms a channel, the

channel erodes

upslope: headward

erosion.

The pattern of the

small streams

(tributaries) that feed

the main stream is

based on the material

being eroded.Fig. 17.3

Base Level

The ability of a stream to

erode is based on velocity

of water. Velocity is

proportional to slope.

At a sufficiently low slope, streams will run without

eroding: this is called the base level. The ultimate base

level is sea level, although local base levels exist, since

flow paths are seldom one consistent slope.

Drainage Basin and Longitudinal Profile

Fig. 17.5

Change in Profile = Change in Base Level

Fig. 17.18

Base Level Changes

lower base level =

steeper profile;

increase stream erosion

(down cut, incise)

raise base level =

shallower profile;

deposit sediment

Processes involving

base level increase:

-- sea level rise

-- uplift at mouth of stream

-- subsidence at head

-- creation of lakes

Processes involving

base level decrease:

-- sea level drop

-- subsidence at mouth

-- uplift at head of stream

-- draining of lakes

Damming

Base level is constantly in

flux, naturally and

artificially.

Dams create artificial

local base levels, with

shallower slopes

(sediment deposited)

upstream and steeper

slopes (erosion)

downstream.

Water Dynamics

Stream sediment

moves by rolling

along the bed or by

bouncing

(saltation).

Stream Transport and Deposition

200 cm/s ~ 4.5 mi/hr

Stream ErosionProcesses by which

streams erode include:

•scouring

•abrading

•lifting/breaking

•dissolution

Darcy’s Law and Discharge

discharge = area x velocity

If discharge increasesand area is unchanged, velocity

must increase.

Internal Features of Stream Channels

Turbulent water flow leads to the

classic meandering path. In this

configuration, water velocity

through the channel is not equal.

Meandering Streams

Immature Streams

Streams draining areas of high slope tend to have such heavy

sediment loads and variable flows that they do not have the

chance to develop organized meandering channels. Instead

they are braided streams. By definition they are not long-lived.

Braided Streams

braided stream typical of those draining melting

glaciers in Alaska

Upper Brahmaputra river, India, draining the immense Himalayan mountains

Alluvial Fans

Alluvial fans occur in areas where steep slopes meet regions of essentially no slope. This causes an immediate deposition of all

sediment, resulting in the characteristic fan shape.

Deltas By fundamental

principle, when a

stream reaches

a large body of

water and

velocity drops,

the material

being

transported is

deposited.

The sediment deposited here is called a delta.

The coarsest-grained material drops out immediately, and finer grained sediment carries out slightly farther,

creating a characteristic deposit.

Modern Delta Sediment

p.302

Delta-related

wetlands are

under stress

from sediment

starvation and

lack of natural

flooding.

Distributaries in deltas periodically shift to

follow the most efficient path to the sea --

that with the steepest slope. This is not

allowed to happen in today’s heavily-

engineered Mississippi delta.

Floodplains

Floodplains form in basins carved from unconsolidated sediment

or soft rocks. The floodplain doesn’t meander like the channel: it is

the broad area of uniform topography around the stream.

Like lake terraces and coastal terraces, these indicate earlier

floodplain positions for streams which erode their floodplains

due to changes in base level.

Fig. 17.21

Stream Terraces

Streams Locked in Valleys

Streams like these have virtually no (ordinary) floodplains. They

have carved into rock so deeply, that their meandering and other

characteristic

evolutionary features are restricted.

Why do some streams erode bedrock like this?

Antecedent Streams

uplift/compressivedeformation

Superposed Streams

Without knowledge of

the regional geologic

history, differentiating

between an antecedent

and superposed

streams can be difficult.

Drainage Pattern Evolution

Old superposed channels cut

across regional structures.

Younger channels follow

predictable paths through easily

eroded valley rocks.

Drainage Pattern Evolution

Sedimentary rocks with sedimentary structures that show

paleocurrent direction of streams give us further supporting

evidence for plate movement, as in the evolution of drainage in

Africa and South America in the last 300 Myr.

Watersheds

Every stream is defined by an area on the ground where incident

precipitation will all flow into that stream. These drainage basins or

watersheds are separated by topographic highs: divides.

When one drainage basin erodes into another, it is stream piracy.

The Continental DivideIn North America, the continental divide

separates watersheds that drain to the west

from those that drain to the east (and the arctic).

The

Mississippi

and its

tributaries

dominate the

North

American

drainage

system.

Most of stream water in the

US drains into the Gulf of

Mexico.

Practically all of the stream

water in South America

drains into the Atlantic.

Do you see why?

What happens to water in these regions?

Western Hemisphere

Drainage

This is the highly vegetated border along most stream channels, providing the transition between aquatic and land environments.

The Riparian

Zone

Aside from stabilizing a channel (through root systems), the riparian zone acts as a filter for land-derived pollutants, and

provides food, shade and habitat for wildlife around the stream. Conservationists call these riparian buffers.

Importance of the Riparian Zone

FloodsIn natural streams,

floods occur

periodically when

seasonal discharges

grow extremely large

and waters exceed

channel banks.

The first thing that

happens after water

goes over bank is the

velocity drops, and

sediment is deposited.

This makes natural

levees along channels.

Flood Recurrence Intervals: Not Prediction

A graph such as this

can be constructed

for any stream where

data exist. The more

long-term data

available, the closer

to being ‘predictive’

these become.

Flood Data

Floods and Urbanization

surface runoff vs. infiltrationnatural land cover vs. urban area

Houston Flooding, 2001

urbanization

(#4 city in US)

+

near sea level

+

tropical storm season

=

Floods: Good or Bad?Prior to mass urbanization of floodplains, natural floods were

not a problem to society. We now attempt to control floods

on most large stream systems by regulating flow with dams

and similar structures.

Nonetheless, natural flooding is important:

• sediment deposits on

floodplains contribute to the

formation of nutrient-rich

soils

• floodplain wetlands are

important habitats for plants

and animals

CreditsSome of the images in this presentation come from:

Marshak, Earth: Portrait of a Planet (1st ed)Hamblin and Christiansen, Earth’s Dynamic Systems (8th ed)

Press and Siever, Understanding Earth (3rd ed)USGS Bulletin 1471

The Chesapeake Bay Program