Sculpting the Earth’s Surface Surface Water

Hydrologic Cycle Hydrologic Cycle

The Hydrologic Cycle  The Three Fates of Precipitation –Infiltration –Runoff –Evapotranspiration

The Hydrologic Cycle  Infiltration capacity of the soil is controlled by: –Intensity and duration of rainfall –Soil saturation –Soil texture –Slope of the land –Nature of the vegetative cover

Surface Water  Saline Water –Oceans –Bays –Estuaries –Some lakes  Fresh water –Lakes –Rivers  Ground Water –In the Earth, flows through fractures and pores

Rivers  Function: –Move runoff away from land and out to the sea (or lake) –Transport sediments (erosion)

Rivers  Factors that determine velocity –Gradient, or slope –Channel characteristics  Shape  Size  Roughness –Discharge – volume of water flowing in the stream (generally expresses as cubic feet per second)

Rivers  Erosion = transportation of sediments –Dissolved load –Suspended load –Bed load  Streambed sediments = Alluvium

How Streams Move Sediment How Streams Move Sediment

Rivers  Load is related to a stream's –Competence - maximum particle size –Capacity - maximum load –Capacity is related to discharge

Rivers – Profile

Rivers  Stream order: –First-order streams have no tributaries but join to form a second-order stream and so on

Rivers Drainage basins and patterns A divide separates drainage basin Types of drainage patterns

The drainage basin of the Mississippi River

A River’s Journey  Begins as sheetflow –Sheetflow develops into tiny channels called rills –Eventually flows into streams, rivers & lakes

A River’s Journey  Headlands –Vertical erosion –Valleys are narrow & V-Shaped Arroyo Seco in the San Gabriel Mountains, Angeles Crest Highway looking towards La Canada & Los Angeles Photo by S. Leyva © 2006

A River’s Journey  Headlands –Smaller channel = Smaller discharge = Higher velocity –Higher velocity = higher energy = greater competence –Larger particles can be moved (clays through small boulders)  Smaller grain sizes moved downstream  Riverbeds typically contain large boulders Sturveyant Falls Trail, San Gabriel Mountains Photo by S. Leyva © 2001

A River’s Journey  Headlands –Features:  Rapids  Waterfalls  Benches

Forming Stream Terraces Forming Stream Terraces

A River’s Journey  Transition from mountains to flatlands –Stream velocity decreases –Decreased velocity = decreased competence = deposition of load –Features  Alluvial Fans  Braided Streams Source: Hyndman & Hyndman’s Natural Hazards & Disasters (

A River’s Journey  Transition from mountains to flatlands –Braided streams typically develop in areas where:  Sediment is abundant  Water discharge is high and sporadic (i.e. seasonal)  Vegetation is sparse –Braiding occurs due to:  Large, rapid fluctuations in river discharge  Large amounts of coarse sediment  Riverbanks that are easily erodible

Braided Rivers From: Geomorphology from Space. Brahmaputra River, Bangladesh

Braided Rivers Yukon River, Alaska The Yukon channel shows a typical braided pattern as it flows over lowlands of Quaternary alluvium known as the Yukon Flats. In this late summer scene, the river is at high flow, transporting snowmelt from nearby mountain ranges. Approximately 700 km to the west, at its junction with the Koyukuk River, the Yukon no longer has a braided pattern. Instead, meander scrolls and anastomosing side channels dominate the lower Yukon. From: Geomorphology from Space. GEO_PLATE_F-12.HTML

River Sinuosity

A River’s Journey  Flatlands –Lateral erosion –Valleys are wide & flat Source: Hyndman & Hyndman’s Natural Hazards & Disasters (

A River’s Journey  Flatlands –Larger channel = Larger discharge = Lower velocity –Lower velocity = lower energy = decreased competence –Only small grain sizes can be moved (clays and silts)  Smaller grain sizes moved downstream  Riverbeds typically contain sands and gravels –Various river types can develop

River Types From: Miall (1977)

A River’s Journey  Flatlands –Features:  Floodplains  Yazoo tributaries  Backswamps  Meanders  Oxbow lakes

Floodplain, backswamp & Yazoo Tributary

Natural Levees

Meanders

Oxbow Lakes

A River’s Journey  Rivers empty out into lakes or oceans –Decreased velocity = decreased competence = deposition of load  Deltas  Ocean canyons

Deltas in various parts of the world

The Mississippi River Delta

Floods  The Worst Geological Hazard –They are the most costly in terms of life, property, and land –They can occur almost anywhere

Floods  The Worst Geological Hazard –Damage is caused by:  Erosion by flood waters  Impact of water on structures  Sediment deposition  Contamination of surface waters

Floods  Recurrence Interval –Time between floods of the same magnitude –Example: there is 1 in 100 chance that a peak flow of a certain amount (say, 30,000 cf/s) can occur on a river. –Thus, that river is said to have a 100-year interval

Floods  Recurrence Interval –Engineers design for particular recurrence intervals  Streets 2-5 years  Debris dams 5 – 25 years  Upper valley dams 100 years  Levees 100 yrs  Large dams years

Floods  Types of floods –Flash Flooding In Arroyos/washes –River (Regional) Flood –Coastal Flood –Urban Flood –Ice Jam –Dam Failure

Types of Floods  Flash Flooding In Arroyos/washes –Typically caused by:  Intense rainfall  Short period of time  Topography, soil conditions, and ground cover also important

Types of Floods  Flash Flooding In Arroyos/washes –Damage:  Roll boulders  Tear out trees  Destroy buildings and bridges  Scour out new channels  Landslides Sturveyant Falls Trail, San Gabriel Mountains Photo by S. Leyva © 2001

Types of Floods  River (Regional) Flood –Some floods occur seasonally when winter or spring rains, coupled with melting snows, fill river basins with too much water, too quickly. –Torrential rains from decaying hurricanes or tropical systems can also produce river flooding.

River (Regional) Floods  Upstream floods –generally local in extent & short lag times. –result from intense storms of short duration.  Downstream floods –regional in extent & longer lag times –higher peak discharges. –Result from regional storms of long duration or extended periods of above-normal precipitation.

Missouri and Mississippi rivers near St. Louis before (1988) and after peak flood conditions

Types of Floods  Coastal Flood –Often caused by storm surges. –Can also be produced by sea waves called tsunamis

Types of Floods  Urban Flood –Roads and parking lots prevent infiltration of water –Urbanization increases runoff 2 to 6 times over what would occur on natural terrain. –Streets can become swift moving rivers, can flood homes and businesses

Types of Floods  Ice Jam –Floating ice can accumulate at a natural or man-made obstruction and stop the flow of water.

Types of Floods  Dam Failure –St. Francis Dam 3/12/28  Dream of William Mulholland  200 ft high curved concrete gravity dam  Failed catastrophically at midnight upon first filling Source: _Dam/franmayn.htm _Dam/franmayn.htm

This colorized photo shows the St. Francis Dam on its day of formal dedication in May of 1926, almost two years before the failure.

Photo, looking north, shows what was left of the St. Francis Dam shortly after it failed on March 12, 1928.

Determination of Past Events

Societal Responses to Floods  Hard Responses –Dams –Levees –Sandbagging  Soft Responses –Watershed Management  Zoning and Land Use  Insurance  Erosion Control –Forecasting

Dams Created by S. Leyva © 2006

Levees

Levees

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