Streams and Floods: the Geology of Running Water

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Chapter 14 Streams and Floods: The Geology of Running Water

Streams and Floods: The Geology of

Running Water

Updated by: Based on slides prepared by:

Rick Oches, Professor of Geology & Environmental Sciences Ronald L. Parker, Senior Geologist

Bentley University Fronterra Geosciences

Waltham, Massachusetts Denver, Colorado

Stream Flow

 Streams and rivers—concentrated flows of water in channels.  A flood occurs when a stream exceeds channel capacity.  Flooding can claim lives and destroy property.  Stream flow is crucial for humans:  Drinking  Irrigation  Industrial use  Transportation  Recreation  Aesthetics  Energy

Fig. 14.1

 Stream runoff is an important geologic agent.  Flowing water: Erodes, transports, and deposits ions and sediments. Sculpts landscapes. Transfers mass from continents to ocean basins.

Fig. 14.19b

Essentials of Geology, 4th edition, by Stephen Marshak © 2013, W. W. Norton Chapter 14: Streams and Floods: The Geology of Running Water The Hydrologic Cycle  Stream flow is part of the hydrologic cycle.  Major parts of the hydrologic cycle:  Evaporation  Transpiration  Precipitation  Sublimation  Infiltration  Melting  Runoff

Geology at a Glance

 Stream flow begins as scattered runoff.

Fig. 14.2

 The configuration of tributaries and trunk streams defines a drainage network.  Geometry is influenced by topography, contrasts in rock types, jointing, and differences in chemical weathering and erodability.

 Common drainage patterns  Dendritic—branching or tree-like  Common in regions of uniform material

Fig. 14.4

 Common drainage patterns  Radial—draining in all directions away from a point  Found at the perimeter of a high region or feature (mesa, peak, or mountain, etc.)

Fig. 14.4

 Common drainage patterns  Parallel—several streams with parallel courses  Common in surface with uniform slope

Fig. 14.4

 Land areas drain into a trunk stream or body of water.  A watershed is the area of land that drains into a stream.  Ridges and peaks separate drainage basins.

Fig. 14.5a

 Watersheds exist at multiple scales.  Large watersheds contain many smaller watersheds.  Continental divides separate drainages that flow to different water bodies (e.g., to Gulf of Mexico versus Atlantic Ocean).

Fig. 14.5b

 Permanent streams:  Ephemeral streams:  Water flows all year.  Dry up part of the year.  At or below water table.  Above the water table.  Humid or temperate climes.  Dry climates. Sufficient rainfall. Low rainfall. Lower evaporation. High evaporation.

Fig. 14.6a, b

 Flow may be slow and smooth or…  Turbulent, the twisting, swirling motion of a fluid. • Uneven velocity • Swirling and eddies  Turbulence caused by: • Irregularities in bank and bottom. • Objects in water. • Steep gradients.

Fig. 14.7b

 River erosion and sediment transport  The energy of flowing water is from mass and gravity.  Streams convert potential energy (PE) to kinetic energy (KE). KE (e.g., fast water flow) lifts and moves solids.  Erosion is greatest during a flood because KE is higher:  Erodes more  Transports more

 Sediment load = material moved by running water load.  Three types of load:  Dissolved load—ions from chemical weathering  Suspended load—fine particles (silt and clay) in the water.  Bed load—larger particles roll, slide, and bounce along the bottom Bed load moves by saltation.

Fig. 14.8c

 Flowing water moves sediments  Slow water can only transport fine sediments and solutes  Fast-flowing water can move boulders and clasts  Competence: a measure of ability to move large clasts

Fig. 14.9a Sediment Deposition

 Decrease in water velocity affects sediment transport.  Competence reduced, sediment drops out.  Boulders, then gravels, then sands fill channel bottoms.  Sands form inside banks (point bars).  Silts and clays drape floodplains.

 Deposition in stream and river systems  Fluvial deposits are sediments transported by streams.  Alluvium—another commonly used term for stream sediments  Sediment accumulates in channel bars and point bars.  During a flood, sediment accumulates on the floodplain.

Fig. 14.9b

 The longitudinal profile describes changes in a stream channel’s elevation from its mouth to its headwaters.  In profile, the gradient describes a concave-up curve. Stream Gradient = change in elevation per distance flowed.

Common units: (ft/mi) = feet of elevation change per mile of channel.

Fig. 14.10

 The character of a stream changes along its length.  Near the headwaters (source) of the stream:  Gradient is steep.  Discharge is low.  Competence is high (sediments are coarse).  Channels are straight.

 Variations in resistance to erosion may result in a stair- step profile along canyon walls.  Strong rocks yield steep slopes and cliffs.  Weak rocks yield gentle slopes.

Fig. 14.11c

 If base level rises, stream flow slows, and the valley become filled with more sediment.  If base level falls, stream flow quickens, and the incision of channels may leave stranded terraces.  Stream terraces mark former floodplains. Fig. 14.12a, b

 Rapids are caused by turbulent flow of water.  Rapids reflect steep gradient and/or high friction.  Flow over bedrock steps (edges of resistant layers)  Flow over large clasts  Narrowing of a channel  Increase in gradient

Fig. 14.13a

 Gradient is so steep that water cascades or free falls.  Waterfalls scours a deep plunge pool.  Basal erosion leads to collapse of overlying rocks.  Waterfalls are temporary base levels.

Virginia Falls, Nahanni National Park, NWT

 Niagara Falls  Lake Erie drops 55 m at the falls, to reach Lake Ontario.  Dolostone cap resists erosion; underlying shale erodes.  Blocks of unsupported dolostone collapse and fall.  Niagara Falls erodes upriver toward Lake Erie.  Erosion since deglaciation has formed Niagara Gorge.

Fig. 14.14

 Alluvial fans build at canyon mouths.  Sediment drops out as water spreads out from mouth.  Coarsest material is dropped first, close to mouth.  Finer material is carried further, to distal edge of fan.  Sediment creates a conical, fan-shaped structure.  During strong flood, debris flow spreads across, smoothing fan surface.

Fig. 14.15a

 Trunk stream consists of many interfingering channels.  Sediment load is very high and channel is very shallow.  Abundant coarse sediment moves during floods.  Sediment is deposited, chokes channel in normal flow.  Weaving channels create ephemeral sand and gravel bars.

Fig. 14.15b

 Have sinuous, looping curves (meanders). They form where:  the stream gradient is low.  the substrate is soft and easily eroded.  the stream exists within a broad floodplain.  Meanders evolve during times of flood.

Fig. 14.16a

 The channel is modified during periods of flood.  Fast part of current swings back and forth.  Momentum increases during flood, erodes outside bank. Curves migrate downstream with time. Fast water erodes the outside stream bank. Slower water deposits point bars on inner part of curve.

Fig. 14.16b

 Curves migrate downstream during flood events.  Sinuosity increases.  Floodwaters may overtop or cut an outside bank to create a shorter pathway downstream, cutting off meander neck.  Oxbow lake is formed from cutoff meander.

Fig. 14.16b

Essentials of Geology, 4th edition, by Stephen Marshak © 2013, W. W. Norton Chapter 14: Streams and Floods: The Geology of Running Water Meandering Streams  Occupy only a small part of the floodplain.  Floodplains are often bounded by bluffs.  During floods, entire floodplain may be immersed.  Natural levees form ridges parallel to the channel.

Fig. 14.16c

Essentials of Geology, 4th edition, by Stephen Marshak © 2013, W. W. Norton Chapter 14: Streams and Floods: The Geology of Running Water Deltas  A delta forms when a stream enters standing water.  Stream divides into a fan of distributaries.  Velocity slows; sediment drops out.  Delta morphology is a dynamic balance of sediment load, waves and storms, tides and slumping.  Bird’s-foot  Arc-like  Δ-shaped

Nile

Mississippi Congo

Fig. 14.17

 The Mississippi is a sediment-dominated delta.  Lobes indicate long-lasting distributaries.  Active lobes grow in size and elevation.  In flood the river may break through a levee—avulsion.  Flow jumps to a more direct path to the Gulf of Mexico.

Fig. 14.18

Essentials of Geology, 4th edition, by Stephen Marshak © 2013, W. W. Norton Chapter 14: Streams and Floods: The Geology of Running Water Drainage Evolution  Stream piracy  One stream captures the flow of another.  A stream with vigorous headward erosion and steeper gradient intercepts a stream with gentler gradient.  The captured stream flows into the new stream.

Fig. 14.20

 Superposed streams flow across deformed terrain despite geologic structure (such as folds).  Stream channel is initially in flat, uniform strata.  Downcutting intercepts underlying, more durable rocks.  Stream may maintain its initial location.

Fig. 14.21

 Antecedent streams  Tectonic uplift may raise a region that has an entrenched stream.  If erosion keeps pace with uplift, the stream remains in its channel.  This is called antecedent drainage.  If the rate of uplift exceeds erosion by the stream, drainage changes.

Fig. 14.22

 Some antecedent streams have incised meanders.  Meanders initially develop on a gentle gradient.  Uplift raises the landscape (resetting the base level).  The meanders erode into the uplifted landscape.

Fig. 14.19b

 Floodwaters devastate property and ruin buildings.  Floods occur when flow exceeds channel capacity.  Water overflows the channel onto adjacent land.  Fast-moving water from channel flows onto floodplain, slows down and drops sediments, sometimes forming natural levees.

Fig. 14.23b

 Flash floods are triggered by various conditions:  Torrential rains rapidly dumping large volumes of water Precipitation > infiltration = runoff.  Long period of continuous rain; ground is saturated  Rapid snowmelt across a large drainage basin  Failure of dam or levee

Fig. 14.24b

 Seasonal floods occur during a “wet season.”  Rainfall is heavy or snow begins to melt.  Tropical regions during monsoon season  Temperate regions when heavy winter snowpack melts  Cause floodplain floods or delta-plain floods.

Fig. 14.23c

Essentials of Geology, 4th edition, by Stephen Marshak © 2013, W. W. Norton Chapter 14: Streams and Floods: The Geology of Running Water Raging Waters  Case history: Mississippi and Missouri Rivers, 1993  Excess rainfall and snowmelt across region  Summer floodwaters invaded huge areas. Covered 40,000 mi2 Flooding lasted 79 days 50 people died 55,000 homes destroyed  $12 billion in damage

Fig. 14.23a

 Flash floods—water rises rapidly with little warning.  Caused by intense rainfall or dam failures  Sometimes a rapidly moving wall of debris-laden water  May strike with little warning and be very destructive Johnstown, Pennsylvania, 1889 (dam failure).

Fig. 14.24a

 Case history: Big Thompson Canyon, Colorado, 1976  Torrential rains (7.5 inches in an hour)  High, fast water scoured rocks and soil  Houses, bridges, and roads vanished; a 275-ton rock moved.  144 people died.

Fig. 14.24b

 Flood control is expensive and ultimately futile.  Levees and flood walls prevent overflow to floodplains.  Artificial levees transmit flood problems downstream.  Levees may be overtopped or undermined.  1993, Mississippi River  2005, New Orleans

Fig. 14.25a, b

 Flood risks are calculated as annual probabilities.  Recurrence interval is the average number of years between floods of a particular size.  A 100-year flood means 1% risk of such a flood in 1 year.

Fig. 14.26b

 Collect hydrologic data and make flood-hazard maps 1% annual probability (100-year floods) 0.2% annual probability (500-year floods)  Maintain flood-control structures

Calgary 2013

Since 1892, there have been two larger floods in Calgary Vanishing Rivers

 Over time, humans have been overusing/abusing rivers:  Pollution  Dam construction  Overuse of water  Urbanization & agriculture

The Colorado River doesn’t reach the sea

Fig. 14.27 Photo by: Osvel Hinojosa-Huerta

 American Water Resources Association  http://www.awra.org/  FEMA On-Line Flood Maps  http://msc.fema.gov/webapp/wcs/stores/servlet/FemaWelcomeView?storeId=10001&catalogId=10001&langId=-1  Hydrologic Research Center  http://www.hrc-lab.org/  National Weather Service Climate Prediction Center  http://www.cpc.ncep.noaa.gov/products/predictions/threats/  NOAA Flood Monitoring  http://www.noaawatch.gov/floods.php  NOAA National Weather Service Flood Safety Program  http://www.floodsafety.noaa.gov/  USGS National Water Quality Assessment Program (NAWQA)  http://water.usgs.gov/nawqa/  USGS Real-Time Water Data for the Nation  http://waterdata.usgs.gov/nwis/rt  USGS Surface Water Information  http://water.usgs.gov/osw/  USGS Water Resources of the United States  http://water.usgs.gov/  USGS Water Watch  http://waterwatch.usgs.gov/new/?m=flood,map&r=us&w=real,map

 Ronald L. Parker, slides 8, 9, 10, 11, 12, 13, 19, 20, 24, 27, 28, 29, 31.

This concludes the Norton Media Library PowerPoint Slide Set for Chapter 14

Essentials of Geology 4th edition (2013) by Stephen Marshak

PowerPoint slides edited by Rick Oches Associate Professor of Geology, Bentley University, Waltham, Massachusetts