Rivers and Floods

The fluvial system - tha major sculptor of landscapes

Rivers and streams are among the main ways that water and sediments reach the sea - concerned with the runoff part of the hydrologic cycle

Drainage and drainage networks

Initial drainage is as sheet flow , but quickly develops into dendritic drainage network of channels typical of all river systems.

Streams rarely slpit going downstream

Rivers join at a confluence.

Drainage basins

All water passing by a particular point is runoff from the basin or catchment area above that point in stream.

Drainage basins consist of sub-basins

nested inside the main basin,

Separated by divides

Flow in rivers and streams

Laminar flow - in laminar flow, water particles move in parallel paths

Turbulent flow - water particle paths cross

Flow in a channel

slower at marging of stream channel, faster in center; but there is an average flow

Discharge

Volume of water passing per unit time (seconds, hours, days)

Discharge = (cross sectional area of channel) x average flow

Examples:

Floods and flood hazard

Floods occur when discharge exceeds capacity of channel due to influx of water into drainage basin - precipation, melting snow etc. Major geologic hazard throughout human history - Noah etc

Discharge with time - flood pulse

Re-occurrence times of floods

Benefits of floods ! !

Sediment tht river carries - classic example Nile

Also schistosomiasis problem now that floods no longer regular

Transport of sediment in rivers

Loads: suspended and bed load (rolling, saltation).

Ability to carry suspended load of given size - competance

Total sediment load - capacity

both greater in turbulent streams

Erosion of bedrock by streams

Abrasion (sand blasting) by bed load this cuts down and deepens channel - evidence in pot-holes found in mountain streams

Channel widens by mass wasting processes that will drop debris into river to be swept away

Deposition of sediment in river systems

Dominant in lower, slow moving part of rivers

River sediment - Alluvium

Sudden change in slope of chennel, hence

of speed of flow(eg. river emerging from mountains) ---> dropping of sediment --> braided stream

Floodplains

Meanders, oxbow lakes (Mark Twain wrote about these in "Life on the Mississippi")

Levees

Where rivers meet the sea

Chemical and bilogical flocculation (surprisingly, legal in Florida)

Estuaries

Deltas

birds foot; shifting channels