300rivers

The Hydrologic Cycle and Rivers

I. The Hydrologic Cycle

I. The Hydrologic Cycle

Water on the earth moves downward by:
- Precipitation = rain and snow
- Surface runoff = rivers and glaciers
- Subsurface infiltration = groundwater
Water vapor moves upward by:
- Transpiration (from plants)
Precipitation + runoff + infiltration = evaporation + transpiration
Water is considered a renewable resource because the hydrologic cycle recycles water so rapidly.

Source for Diagram: http://www.sunynassau.edu/users/fanellis/oceanology/hydrocycle.htm

II. Factors Controlling the Flow of Rivers

Velocity of the water - measured in feet per second. The faster the velocity, the larger the particles that will move.
Gradient of the channel - measured in vertical feet dropped per mile of channel length. The steeper the gradient, the greater the velocity will be.
Size of the channel - measured in square feet; the area of the cross section of the channel. The larger the size, the grater the velocity will be.
Roughness of the channel - not a measured value. The smoother the channel, the greater the velocity will be.

III. Erosion and Transportation

Erosion - the removal of weathered material.
Transportation - downhill movement of weathered material.
Agents of erosion and transportation include gravity (rock slide), water (rivers or waves), ice (glaciers), or wind (dust storms).
Load - earth material carried by the agent of transportation.
- Dissolved load - material dissolved in the agent and carried in the ionic state.
- Suspended load - particles that are carried by the agent and that never touch the bottom (silt and clay in rivers).
- Bed load - particles carried by the agent that bounce or roll along the bottom (sand and gravel in rivers).
Cross-sectional shape of erosional river or stream valleys -
- In the headwater regions, rivers erode downward faster than sideways, so with the help of mass wasting on the side slopes, they produce V-shaped valleys.

IV. Deposition

When running water slows down or comes to a stop, the load that it is carrying is dropped (deposited) on the bottom.
The material that is deposited by rivers and streams is called alluvium.
Types of alluvial deposits -
- Alluvial fan - a cone-shaped deposit that occurs where a mountain stream flows out onto a flat valley floor.
- Channel of a meandering river - a sinuous deposit of relatively coarse alluvium that is surrounded by a wide expanse of relatively fine deposits.
- Natural levees of a meandering river - deposits of relatively intermediate grain size that form along the sides of the channel deposits during flooding as a result of water leaving the channel and slowing down as it spreads away from the channel.
- Floodplain of a meandering river - a wide, flat expanse of relatively fine-grained sediment that is deposited beyond the natural levees during periods of flooding.
  
  Source for Diagram: http://www.dot.co.pima.az.us/flood/fpm/
- Cutoff - Shortening of a meander curve by establishment of a new channel during a period of flooding.
- Oxbow lake - a lake formed in an abandoned meander curve after a cutoff has established a new channel.
- Plain of a braided river - a wide expanse of relatively coarse-grained alluvial deposits.
- Delta - fan-shaped deposit formed where a river enters a lake or the ocean.

V. Implications for man

Urbanization (paving the land) increases runoff from rain and thus promotes flooding.
Lakes and reservoirs are temporary base levels; eventually they will be filled in by delta and alluvial deposits.
River banks are always subject to erosion by the river, especially the outer banks of meander curves, so they are ill suited for permanent construction.
Floodplains are always subject to flooding, so they also are poor places for permanent construction; the river level inside the natural levees is higher than the level of the floodplain.

VI. Possible essay questions -