Chapter 7- Running Water and Groundwater

The Hydrologic Cycle Hydrologic Cycle: circulation of Earth’s water supply among the oceans, the atmosphere and the continents Powered by Sun’s energy Water is transported by: Evaporation – liquid water converts to gas and rises into atmosphere from land or oceans Precipitation – gaseous water in atmosphere condenses back into liquid and drops onto land or oceans Infiltration – water from precipitation soaks into the ground Runoff – water from precipitation flows over ground surface Transpiration – water absorbed by plants is released into the atmosphere

Runoff, 40 Groundwater Flow Infiltration

Sources of Earth’s water

Earth’s Water Balance Balance in the water cycle means: average annual precipitation over Earth equals the amount of water that evaporates. Avg Annual Precip=Amt water Evaporated

Running Water Importance: Stream: channel of flowing water of any size Energy Travel Irrigation Fertile floodplains Shaping of landscape (via erosion) Stream: channel of flowing water of any size River – larger streams Tributaries or brooks – smaller streams

Streams flow because of gravity Streams are supplied by runoff and infiltrating groundwater. Runoff and groundwater come from precipitation. Drainage basin: land area that contributes water to a stream. Divide: imaginary line separating drainage basins.

Stream Velocity Stream Velocity: distance water travels in given time Measured at gauging stations Stream velocity determines erosional capability Three factors affect velocity: Gradient (slope) – expressed as the vertical drop of a stream over a fixed distance. High gradient = high velocity. Channel characteristics (shape, size and roughness) – more on this in next slide. Discharge (volume of water flowing per unit time) – High discharge = high velocity. Discharge also affects channel characteristics.

Velocity varies within the channel Highest velocity is in the center of the channel Drag slows water along the bottom, banks, and top (water-air interface)

Stream Channel Characteristics Channel shape (cross-sectional) affects velocity. Wide, shallow channel = slower (a lot of friction with streambed). Semicircular channel = faster (least amount of friction). Channel size: larger = faster. Channel roughness: smoother = faster. Which stream below is faster? A B

Running water Upstream-downstream changes Profile Cross-sectional view of a stream From head (source) to mouth Profile is a smooth curve Gradient decreases from the head to the mouth Factors that increase downstream Velocity Discharge Channel Size

Running water Upstream-downstream changes Profile Factors that decrease downstream Gradient, or slope Channel roughness

Longitudinal profile of a stream

Running water Base level Lowest point a stream can erode to Two general types Ultimate – sea level Temporary, or local Changing causes readjustment of the stream – deposition or erosion

Adjustment of base level to changing conditions

A waterfall is an example of a local base level

Geologic Work of Streams – Transportation of Sediment Running water is the single most important factor in shaping the Earth’s land surface. By Erosion: Streams erode their channels lifting loose particles by abrasion, grinding, and dissolving soluble materials By Transportation: Water carries sediments in 3 ways: In solution (dissolved load) – mostly transported by groundwater In suspension (suspended load) – most materials carried by streams are suspended loads Along bottom of channel (bed load) – moves only intermittently

Running water The work of streams Transportation Load is related to a stream's Competence - maximum particle size Capacity - maximum load Capacity is related to discharge

Geologic Work of Streams – Deposition of Sediment Streams deposit suspended sediment when they slow down. Recall from unit on rocks: fast streams can carry larger particles, while slow streams can only carry smaller particles. Sorting: particles of similar size are deposited together Alluvium: material deposited by a stream Natural levees: an elevated landform that parallels a stream and acts to confine its waters Alluvial Valleys Deltas Alluvial Fans

Alluvial Valleys Streams fill part of their valleys with sediment Changes in hydrologic conditions initiate deposition Subsequently cut through deposits Create terraces

Deltas Formed by sudden drop in velocity Stream enters ocean or lake Stream drops sediment load Evolve by three mechanisms Growth may be influenced by ocean waves

Delta Growth Distributaries Splays Avulsion Levee and in-stream deposit formation Channel splits into two smaller channels Splays Crevasse allow water to leave main channel Water spreads out forming a splay deposit Avulsion Shift in main course to follow steeper slope

Structure of a simple delta

Alluvial Fans Accumulation of sediment in a dry basin Deposition due to rapid velocity drop Usually arid climate High sediment load - braided streams Form fan-shaped deposit

Alluvial fans in Death Valley

Running water Stream valleys Valley sides are shaped by Weathering Overland flow Mass Wasting Characteristics of narrow valleys V-shaped Downcutting toward base level

Running water Stream valleys Characteristics of narrow valleys Features often include Rapids Waterfalls Characteristics of wide valleys Stream is near base level Downward erosion is less dominant Stream energy is directed from side to side This eventually produces a floodplain-a flat valley floor Streams that flow on floodplains move in meanders

A narrow V-shaped valley

Continued erosion and deposition widens the valley

The resulting wide stream valley is characterized by meandering on a well-developed floodplain

Erosion and deposition along a meandering stream

A meander loop on the Colorado River

Braided Streams Multiple channel system Interlaced channels with islands High sediment load Common in arid & semi-arid regions Seasonal high flows Common in front of glaciers Large sediment load, fluctuating water flow

Running water Floods and flood control Floods are the most common geologic hazard Causes of floods Weather Human interference with the stream system

Floodplains Floods Floodplains Normal stage - water level below the bank Bankfull stage - water level even with the bank Flood stage - water level above the bank Floodplains Areas that will be submerged when a river is at flood stage

Running water Floods and flood control Engineering efforts Artificial levees Flood-control dams Channelization Limiting Development Nonstructural approach through sound floodplain management

Satellite view of the Missouri River flowing into the Mississippi River near St. Louis

Same satellite view during flooding in 1993

Running water Drainage basins and patterns Drainage basin: the land area that contributes water to a stream A divide separates drainage basins of one stream from another Types of drainage patterns Dendritic Radial Rectangular Trellis