Erosion and Transportation River Systems II Erosion and Transportation

Daily Question Which of the following effects does urbanization have on stream systems? Decrease risk of water pollution Increase in discharge Increases flooding intensity and frequency Increases erosive ability of the stream Increase in infiltration to ground water

Urbanization Effects Decrease infiltration – Increases runoff Produces Higher Peak Discharge Produces shorter lag time Produces more floods Higher discharge = increased ability to erode

Erosion Removal and transport of rock, sediment, soil Running water is the major cause of erosion Source of Materials in a stream Mass Wasting Lateral erosion of banks Downward erosion of stream channel Headward erosion Sheetflow Chemical Weathering

Stream Dynamics – Sediment Load Bed Load Suspended Load Dissolved Load

Sediment Load – Bed Load Particles to large to be lifted into suspension Sediment moves along stream bed Particles move by sliding, rolling, or saltating (short leaps) Accounts for 10 to 25% of sediment load Agent of downward erosion

Sediment Load – Suspended Load Light sediment lifted above stream bed by current Mostly silts and clays Accounts for most of stream load ( 60%)

Sediment Load – Dissolved Load Chemical ions produced from chemical weathering of minerals Ca2+, Cl-, Mg2+, SO42-, Na+, K+, HCO3-, etc. Acquired from groundwater, sheetflow, or dissolving rock along a stream’s course. Generally  10%, but can be up to 50% of sediment load.

Effects of Transport on Particles Rounding caused by abrasion Increases with distance particle is transported Sorting Varies with the competence of the stream As competence decreases large particles are deposited

Controls of Sediment Load Capacity The amount (weight) of sediment a stream can carry It is a function of discharge (velocity) Velocity  (proportional to) Capacity 3 to 4 Example, if velocity doubles (2) – capacity increases between 8 (23) to 16 (24) times Competence The maximum particle size a stream can transport Function of velocity and fluid density Velocity  (proportional to) Competence2 Example, if velocity doubles (2) – competence increases 4 (22) times As stream density increases with more suspended and dissolved sediment, competence increases

Fig. 11.16

Velocity’s role in Erosion Threshold Velocity – minimum velocity required to move grains of a certain size

Fig. 11.19

Deposition of Material Occurs when streams lose velocity (competence decreases) Velocity decreases due to Lower gradients Flow into still or slow moving waters (reservoirs, lakes, etc.) Changes in channel shape (widening or narrowing)

Meanders - Cut Banks & Point Bars Fig. 11.20

Meander Development Meander loops migrate laterally and downstream As meanders migrate and streams continues to erode and deposit sediment, a meander can be cutoff creating an oxbow lake. What would happen to the streams gradient?

Longitudinal Profile Cross-sectional image showing the variation in a stream’s elevation along its length

Base Level The downward limit of stream erosion; lowest elevation to which a stream can erode its channel Ultimate Base Level – sea level Local Base Level – lakes, resistant layers of rock, reservoirs, etc.

Effects of Changes in Base Level Changes in base level change the energy of the system, which changes the velocity of the water. Consider the consequences of constructing a dam on a river that has a large stream load, such as the Yellow River, China. Assume the dam and its reservoir are located about 2/3 of the way down the river. How would stream conditions be altered above and below the dam and its reservoir? What would be the implications for erosion, transport, and deposition?