Streams and Floods

Prepared by Betsy Conklin for

Dr. Isiorho

The Hydrologic Cycle

• the movement of water and water vapor from the sea to the atmosphere, to the land, and back to the sea and atmosphere

The Hydrologic Cycle (cont.)

Streams• stream: a body of running water that is confined in a

channel and moves downhill under the influence of gravity

• can imply size (rivers = large, streams = smaller, creeks = smallest) but geologists use the term when talking about any body of running water

• headwaters of stream: upper part of stream near its source in mountains

• mouth: place where a stream enters the sea, lake, or larger stream

Streams (cont.)• flood plain: a broad strip of land built by sediment

on either side of a stream

• stream channel: a long narrow depression eroded by the stream into rock or sediment

• stream banks: the sides of the channel

• stream bed: the bottom of the channel

• sheetwash: a thin layer of unchanneled water flowing downhill, common in deserts due to lack of vegetation

Streams (cont.)• sheet erosion: when a thin layer of surface material,

usually topsoil, is removed by a flowing sheet of water

• rills: the tiny streams formed by overland sheetwash that becomes concentrated in small channels

• longitudinal profile: shows a typical stream viewed from the side

• cross section: a representation of a portion of a stream in a vertical plain

Drainage Basins• drainage basin: the total area drained by a stream

and its tributaries

• can be outlined on a map by drawing a line around the region drained by all the tributaries to a river

• tributary: a small stream flowing into a larger one

• divide: a ridge or strip of high ground dividing one drainage basin from another

Drainage Patterns

• drainage pattern: the arrangement of a river and its tributaries

• types:– dendritic: resembles the branches of a tree or

veins in a leaf

Drainage Patterns (cont.)

– radial: when the streams diverge outward like spokes of a wheel; formed on high conical mountains

Drainage Patterns (cont.)

– rectangular: when tributaries have frequent 90o

bends and tend to join other streams at right angles; develops on regularly fractured rock

Drainage Patterns (cont.)

– trellis: consists of parallel main streams with short tributaries meeting them at right angles; forms in a region where tiled layers of resistant rock such as sandstone alternate with nonresistant rock such as shale

Drainage Patterns (cont.)

Stream Velocity

• stream velocity: the distance water travels in a stream per unit time

• moderately fast river = 5 km/hr (3 mph)

• stream reaches its maximum velocity near the middle of the channel

• high velocity generally results in erosion and transportation

• low velocity generally results in deposition

Stream Velocity (cont.)

• factors effecting stream velocity:– shape of the channel– roughness of the channel– human interference (construction of a culvert or

bridge can partially block a channel, increasing a stream’s velocity)

Stream Gradient

• stream gradient: downhill slope of a stream’s bed or the water surface, if the stream is very large

• gradient of 5 ft/mile means the river drops 5 ft. vertically for every mile that it travels horizontally

• usually decreases downstream

Discharge

• discharge: the volume of water that flows past a given point in a unit of time

• found by multiplying the cross-sectional area of a stream by its velocity (or width X depth X velocity)

• reported in cubic feet per second (cfs) in the US, or in cubic meters per second (m3/sec)

Discharge (cont.)

• increases during a flood, may be up to 50 to 199 times normal flow

• stream erosion and transportation generally increase as a result of a flood’s velocity and discharge

Stream Erosion• three types of stream erosion:

– hydraulic action: refers to the ability of flowing water to pick up and move rock and sediment

– solution: usually slow but effective process of weathering and erosion in which rocks are dissolved by water

– abrasion: the grinding away of the stream channel by the friction and impact of the sediment load; can cause potholes (usually most effective on a rocky stream bed)

Stream Transportation of Sediment

• bed load: the large or heavy sediment particles that travel on the stream bed

• traction: movement of sediments by rolling, sliding or dragging

• saltation: when sediments are carried downstream in a series of short leaps or bounces

Transportation of Sediment (cont.)• suspended load: sediment that is light enough

to remain lifted indefinitely above the bottom by water turbulence; the muddy appearance of a stream during a flood or after a heavy rain is due to a large suspended load

• dissolved load: the portion of the total sediment load in a stream that is carried in solution

Bar• bar: a ridge of sediment, usually sand or

gravel, deposited in the middle or along the banks of a stream; formed by deposition when a stream’s discharge or velocity decreases; stream deposits heavier boulders first and small particles last

Gravel bars along the banks and in the middle of a stream

Placer Deposits

• when the heavy sediment is concentrated in the stream where the velocity of the water is high enough to carry away lighter materials but not the heavy sediment

• found in streams where the running water has mechanically concentrated heavy sediment

Braided Streams• a stream that flows in a network of many

interconnected rivulets around numerous bars

• a stream tends to become braided when it is heavily loaded with sediment and has banks that are easily eroded

A braided river

Meandering Streams and Point Bars• meanders: a pronounced sinuous curve

along a stream’s course

• point bar: a stream bar deposited on the inside of a curve in the stream, where the water velocity is low

• meander cutoff: a new, shorter channel across the narrow neck of a meander

A meandering stream

Meandering Streams/Point Bars (cont.)• the simultaneous erosion on the outside of a

curve and deposition on the inside can deepen a gentle curve into a hairpin-like meander

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Deposition Erosion

Erosion Deposition

Point bars

Curve shifts outwardand downstream

Oxbow Lake• oxbow lake: a crescent-shaped lake

occupying the abandoned channel of a stream meander that is isolated from the present channel by a meander cutoff and sedimentation

Meanderneckbecomesnarrower

Neck cutoffoccurs

Oxbow lake

Flood Plains

• flood plain: a broad strip of land built up by sedimentation on either side of a stream channel

• natural levees: low ridges of flood-deposited sediment that form on either side of a stream channel and thin away from the channel

Flood Plain (cont.)1 2

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Flood Plains

Sediment deposited during floods

Natural leeves Backswamp

Deltas• delta: a body of sediment deposited at the mouth

of a river when the river’s velocity decreases• distributaries: small, shifting channels that carry

water away from the main river channel and distribute it over the surface of the delta

• a delta can be represented by the Greek symbol delta () due to the fact that some deltas are broadly triangular

Deltas (cont.)

• foreset beds: form the main body of the delta

• topset beds: nearly horizontal beds of varying grain size formed by distributaries shifting across the delta surface

• bottomset beds: deposits of the finest silt and clay that are carried out into the lake by the river water flow or by sediments sliding downhill on the lake floor

LandRiver

Topset bedsForeset beds

Distributaries

Marshy delta surfaceLake

Bottomset beds

Deltas (cont.)

• wave-dominated delta: a delta formed by the reworking of sand by wave action

Mediterranean Sea Distributaries

Barrier Islands

Nile River(Egypt)

Deltas (cont.)• tide-dominated delta: a delta formed by the

reworking of sand by strong tidesBangladesh

Tidal sand bars

Deltas (cont.)• stream-dominated delta: a delta with finger-

like distributaries formed by the dominance of stream sedimentation (also called birdfoot delta)

Alluvial Fans• alluvial fan: a large, fan- or cone-shaped

pile of sediment that usually forms where a stream’s velocity decreases as it emerges from a narrow mountain canyon onto a flat plain

Narrow mountain canyon

Braidedstream

Alluvial fan

Plain

Flooding

• recurrence interval: the average time between floods of a given size

• flood erosion: caused by the high velocity and large volume of water in a flood

• high water: covers streets and agricultural fields and invades buildings, shorting out electrical lines and backing up sewers

• flood deposits: usually silt and clay

Urban Flooding• urbanization enhances flooding due to paved

areas and storm sewers which increase the amount and rate of surface runoff of water, making river levels higher during storms

• bridges, docks, and buildings built on flood plains can also constrict the flow of flood waters, increasing the water heigh and velocity and promoting erosion

Flash Flooding

• flash floods: local, sudden floods of large volume and short duration, often triggered by heavy thunderstorms

Controlling Floods• upstream dams: trap water and release it slowly

after a storm• artificial leeves: embankments built along the banks

of a river channel to contain floodwaters within the channel

• protective walls: made of stone (riprap) or concrete constructed along river banks to slow erosion

• floodwalls: walls of concrete, may be used to protect cities from flooding

Controlling Floods (cont.)• bypasses: reduce the discharge in the main

channel by diverting water into designated basins in the flood plain

ReservoirBypass

Flood wall

Dam

Leeve

Weir

Stream Valley Development

• valleys: the most common landforms on the earth’s surface that are usually cut by streams

• downcutting: the process of deepening a valley by erosion of the stream bed

• base level: the limit of downcutting

Graded Stream• graded stream: a stream that exhibits a

delicate balance between its transporting capacity and the sediment load available to it

• an increase in gradient causes an increase in a stream’s velocity, allowing the stream to erode and carry more sediment

• a change in sediment load can cause a change in gradient as well

Lateral Erosion• lateral erosion: the erosion and undercutting

of a stream’s banks and valley walls as the stream swings from side to side across its valley floor

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Undercutting ofvalley wall

Wideningflood plain

Headward Erosion & Stream Piracy

• headward erosion: the slow uphill growth of a valley above its original source through gullying, mass wasting, and sheet erosion

• stream piracy: the natural diversion of the headwaters of one stream into the channel of another

Stream Terraces

• stream terraces: steplike landforms found above a stream and its flood plain

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Flood plain Terraces

TerracesNew flood plain

Terraces

Incised Meanders• incised meanders: meanders that retain their

sinuous curves as they cut vertically downward below the level at which they originally formed resulting in a meandering valley

Meandering river

Base level

Land surfaceat base level

Land surface has beenlifted above base level

Incised meanders

Base level

Uplift

Incised meanders of the Colorado River

Superposed Streams• superposed streams: a river let down onto a

buried geologic structure by erosion of overlying layers

The stream initially cuts throughthe horizontal sediment

Continued erosion removes horizontal strata and stream cuts throughunderlying rock formingnarrow valleys in resistant rock

Pictures

All pictures used in this power point presentation were taken from the following:

Carlson, Diane H., David McGeary and Charles C. Plummer. Physical Geology: Updated Eighth Edition. New York City, McGraw-Hill Higher Education, 2001.