chapter 15- running water

Chapter 15- Running WaterRUNNING WATEREarth is the water planet, unique in our Solar System:

-97% of water on Earth is in the oceans-2% is frozen in glaciers-rest is in rivers, lakes, groundwater, etc

Streams and Rivers- i.e. running water - are important for several reasons:-erosional agents-transportation agents-economic uses

Hydrologic Cycle: moving water is a major part in this cycle: re-circulationof water from oceans to atmosphere to continents to oceans, and repeat….

Transportation Aspect: fluid flow occurs in two forms- 1.laminar flow- sheet like, planar; occurs at shallow depth; less common; groundwater flow 2. turbulent flow- irregular flow due to velocity and roughness; more common

Chp15: Running WaterHydrologic Cycle: water evaporates, rises as water vapors, forms clouds,Rain, washed to ocean thru rivers…process repeats…

Chp15: Running Water

Relative amounts of water on Earth

Running Water is the most important geologic agent modifying the Earth’s surface. Exception- glaciers and deserts. 1. sheet flow vs channelized flow:

-sheet flow: not confined to depressions -channelized flow: is confined to trough-like depressions. Width and depth varies- from creek to large rivers

2. Stream gradient: flow downhill along a slope…the gradient is measured by the vertical drop / horizontal distance: 1000m/500km= 2m/km

3. Velocity and Discharge: a. Velocity: downstream distance water travels in time: m/sec or ft/sec -Varies across channel width and along its length: water moves slower along riverbed and at banks-friction is higher. -Channel shape and roughness: water flows faster in semicircular channel profile than others; less contact with bed and banks b. Discharge: volume of water passing any point in given period of time: discharge (Q) =cross sectional area (A) * flow velocity (V) Q= A * V

Mississippi River= 18,000 cubic cm/ sec Amazon River= 200,000 cubic cm/sec

Chp15: Running WaterLaminar flow-streamlines parallel to one another

Turbulent flow-streamlines intertwined, mixing occurs

Chp15: Running Water

Average gradient of stream= 2m/kmCan vary over length of stream…

Erosion and Transportation by Running Water

Erosion removes loose particles of rock and soil, including dissolved substances (dissolved load in streams)

Solid particles: moved by several processes- 1. abrasion: rock is worn and scraped by impact of solid particles 2. bed load: larger particles of sand and gravel size move along stream bed. 3. dissolved load: materials put into solution by chemical weathering- usually fine sized sedimentary particles.

Suspended load- silt and clay sized particles kept suspended by turbulence.

a. Sediment transported as bed load and suspended load. Arrows on right are proportional to water velocity.b. Particle size and velocity of moving water influences sediment erosion, transportation and deposition

Deposition by Running Water Alluvium= sediments deposited when a river floods.

1. Braided Stream Deposits: numerous channels separated by gravel/sand bars When viewed from above, look like braids -Braided streams develop when sediment volume exceeds transport capacity. - Usually deposit coarse material (sand and gravel). Common in arid and semi arid regions.

2. Meandering Streams: single, sinuous channel with broad loops/curves known as meanders. -semi circular stream bed profile in cross section in straight regions; asymmetric around curves. -due to higher velocity on outer bank (cut bank)- cut bank erodes, with deposition on opposite bank. These deposits are called point bars. These are sand-rich: well sorted, often cross bedded!

Oxbow lakes: meanders cut-off during river flood, left behind-isolated- after water level recedes.

Braided stream in Alaska

Diagrammatic view of meandering river

a. In a meanderingchannel the velocityis greatest near outerbank. Arrows proportionalto velocity. Outer bank iseroded, inner bank has pointbar deposited…

b. 2 point bars at bends ofmeandering river.

c. This point bar is severalhundred meters across.

Deposition by Running Water

3. Floodplain deposits: when a river floods, it spreads out over its banks along low relief features……known as floodplainfloodplains can consist of any type of deposit: -point bars that build laterally due to lateral accretion (sand rich) -natural levees: during flood, sands pile up along stream margin, building vertically. Fine grained material deposited outside the levee-channel.

Floodplain deposits forming as result of lateral accretion of pointbars.

3 stages of floodplain development by vertical accretion:a. Stream at low water stage.b. Stream floods, deposits levees and mud in overbanks.c. Stream confined to channel, bordered by levees.

4. Deltas: -when running water enters another body of water, its velocity decreases and it deposits sediment -local shoreline moves seaward, or progrades, as sediment is deposited -a distinctive vertical sequence results: bottomset beds, foreset beds, topset beds.3 types of marine deltas are identified: function of erosion and sediment supply: a. stream dominated-(Mississippi R) long finger like features; “birds foot” b. wave dominated- (Nile R) barrier islands around edge of delta; curved c. tide dominated- (Ganges R) tide parallel ridges perpendicular to coast

5. Alluvial Fan:Lobate deposits of alluvium on land; common in arid to semiarid regions with little vegetation.During and after rainstorms, water rushes down canyons, when reaches mouth of canyon, dumps its load quickly (poorly sorted)

A. Internal structure of delta.b. Small delta, about 20macross showing 3 sets

Delta classification:a. Stream dominated- Long finger like extensions(Mississippi R)

b. Wave dominated-arcuate(Nile R)

c. Tide dominated-Linear ridges perpendicular to delta edge.(Ganges R)

5. Alluvial Fan:Lobate deposits of alluvium on land; common in arid to semiarid regions with little vegetation.During and after rainstorms, water rushes down canyons, when reaches mouth of canyon, dumps its load quickly (poorly sorted)

a. Alluvial fan forms where streamsdischarge from mountain canyons ontoadjacent lowlands.b. Alluvial fans in Death Valley, CA

Flood Control We try to control river flooding with dams and levees: this in only viable to certain level which a river will exceed eventually

Chp15: Running WaterOroville Dam in CA: flood control, recreation, crops

Natural falls in river, wouldthis adjust river gradient?

Flood control: levee along river bank constructed to keep river containedduring flood. B. floodways constructed to carry excess water from riversaway from communities.

Significance of the ‘Base Level’ concept: Base Level concept: the lowest level to which rivers can erode… in most cases this is Sea Level. Locally this can be different-if Man has made dams etc. in lakes or streams; or if naturally occurring features act as same… -Note that when sea level rises or lowers, base level changes

Graded Stream concept…. Graded stream is a stream with an equilibrium profile in which a delicate balance exists between gradient, discharge, flow velocity, channel shape, and sediment load so that neither erosion or deposition occur within the river channel.

This condition is closely approximated in many streams and rivers.

Graded streams:

a. An ungraded streamhas irregularities along a longitudinal profile.

b. Erosion and depositionalong stream tend to eliminateirregularities and yielda concave profile approximatinga graded profile.

c. Actual measured profilesof 3 rivers. VE=275x

a. Constructing a dam and reservoir create a local base level.b. Stream will adjust to new base level if lake is drained.

Illustrations of local base levelconcepts in relation to ultimatebase level (sea level)

How do Valleys form? Valley-on land, a low area bounded by higher areas. Gullies-canyons-gorges: steeper sides, deeper valleys1. stream terraces: have a fairly flat upper surface, steep slope descending to lower level of present-day floodplain. -evidence of streams flowing at higher elevation in past (eroded down to present level!!)

2. incised meanders: streams restricted to deep meandering canyons cut into bedrock. -downcutting: occurs when a stream has more energy than it needs to transport its sediment load.

3. superposed streams: streams that are seen to cut through ridges of bedrock (Appalachian Mtns).

Valley of Colorado R in Moab, UT…note steep valley walls.

This river valley has gently sloping walls….

Meandering river becomes incised as it erodes downward.

Origin of superposed stream: stream begins cutting down into horizon-tal strata. Horizontal layer removed by erosion, exposing underlyingstructure. Stream cuts narrow valleys in resistant rocks that form ridgesforming water gaps.

Climate and River flooding: - seasonal impacts that vary from year to year.-Floods actually re nourish the top soil in floodplain, over time increase ability of soils to yield good crops.

-Economic impact in terms of flood control, damage mitigation, etcis considerable.

Dominant weather pattern over US in summer 1993: concentrated rains.howers in Midwest

Satellite photos of 3 rivers flooded in summer ’93. X=Portage, Ill

Damage near Portage, ILL caused by floods in summer1993.

Damage caused by flood of 1993…US Corps of Engineers

Economic Uses: transport materials on river.

Chapter 15- Running Water: SummaryStreams and Rivers- i.e. running water - are important for several reasons:

-erosional agents-transportation agents-economic uses

1. Transportation Aspect: fluid flow occurs in two forms--laminar flow- sheet like, planar; occurs at shallow depth; less common; groundwater flow-turbulent flow- irregular flow due to velocity and roughness; more common

1.sheet flow vs channelized flow: a. sheet flow: not confined to depressions b. channel flow: is confined to trough-like depressions. Width and depth varies-from creek to large rivers

2. Stream gradient: flow downhill along a slope…the gradient is measured by the vertical drop / horizontal distance: 1000m/500km= 2m/km 3. Velocity and Discharge:

Velocity: downstream distance water travels in time: m/sec or ft/sec -Varies across channel width and along its length: water moves slower along riverbed and at banks-friction is higher.

-Channel shape and roughness: water flows faster in semicircular channel profile than others; less contact with bed and banks b. Discharge: volume of water passing any point in given period of time: discharge (Q) =cross sectional area (A) * flow velocity (V) Q= A * VErosion and Transportation by Running WaterErosion removes loose particles of rock and soil, including dissolved substances (dissolved load in streams)1. Solid particles: moved by several processes-

a. abrasion: rock is worn and scraped by impact of solid particles b. bed load: larger particles of sand and gravel size move along

stream bed. c. dissolved load: materials put into solution by chemical weathering-using fine sized sedimentary particles.

Suspended load- silt and clay sized particles kept suspended by turbulence.

Deposition by Running Water: Alluvium= sediments deposited when a river floods.1.Braided Stream Deposits: numerous channels separated by gravel/sand bars2. Meandering streams: curved sinuous streams3. Floodplain deposits: deposits on flanks of rivers, outside river bank4. Deltas: 3types are important: wave, tide, and stream dominated5. Alluvial fans: lobate features formed where canyons exit mountain range

Significance of the ‘Base Level’ concept: Base Level concept: the lowest level to which rivers can erode…in most cases this is Sea Level. Locally this can be different-if Man has made dams etc. in lakes or streams Note that when sea level rises or lowers, base level changesGraded Stream concept…. Graded stream is a stream with an equilibrium profile in which a delicate balance exists between gradient, discharge, flow velocity, channel shape, and sediment load so that neither erosion or deposition occur within the river channel.

How do Valleys form? Valley-on land, a low area bounded by higher areas. Gullies-canyons-gorges: steeper sides, deeper valleys1. stream terraces: have a fairly flat upper surface, steep slope descending to lower level of present-day floodplain. -evidence of streams flowing at higher elevation in past (eroded down to present level!!)2. incised meanders: streams restricted to deep meandering canyons cut into bedrock.-downcutting: occurs when a stream has more energy than it needs to transport its sediment load. 3. superposed streams: streams that are seen to cut through ridges of bedrock (Appalachian Mtns).Economic Uses and Impact of Rivers- transportation, soil replenishment for crops

Chp15: Running Water

Chp15: Running Water