Distribution of Water - Know the Major Reservoirs
Freshwater Resources
| Reservoir | % freshwater | Exchange Rate |
| Ice Sheets & Glaciers | 85 | 8000 years |
| Groundwater | 14 | 280 years |
| Lakes & Reservoirs | 0.55 | 7 years |
| Soil Moisture | 0.3 | 1 year |
| River Water | 0.004 | 11.3 days |
| Atmosphere | 0.05 | 9.9 days |
| Oceans | n/a | 16,500 years |
Hydrologic Cycle
Infiltration Capacity - (does the water go into the ground or stay on the surface ?)
When rain first falls, it infiltrates into the dry soil. When the soil becomes saturated the excess water must run off of the surface. It first runs off as sheet wash, a thin layer of unchannelled water flowing across the surface of the soil.
Eventually, the water will be gathered into small channels called rills. These will join to form creeks and streams which will flow into rivers and finally into lakes and the ocean.
Stream - Any channelized flow of any size from a trickle to a river.
Flow Behavior
- Laminar
- Turbulent
Type of Flow Determined by
Velocity
Laminar flow is typically slow
Turbulent flow results from increased velocity
The ability of a stream to erode/transport material is directly related to its velocity.
Faster - more erosion / more transport
carries larger particles
Slower - less erosion / less transport
carries only smaller particles
Velocity is related to 1. gradient - the amount of vertical drop over a horizontal distance.
Velocity is related to 2. The size, shape and roughness of the channel.
Friction between the channel and the water
Laminar flow requires low friction (smooth channels)
Turbulent Flow results from rough surfaces and higher friction
DISCHARGE - the amount (volume) of water flowing past a certain fixed point in a given unit of time.
Discharge (m3/sec)
= velocity (m/s) x channel width (m) x channel depth (m)
Stream Erosion
The Earth's most important erosional agent!
Streams transport an enormous amount of material produced by weathering and transported to the stream by sheet flow, mass wasting and groundwater flow.
1. Dissolved Load
Mostly supplied by groundwater - soluble compounds (like salt) - products of chemical weathering.
Unaffected by changes in velocity - precipitation occurs only when the chemistry of the water changes.
2. Suspended Load
Visible cloud of particles/sediment suspended in the water - usually fine sand, silt, and clay
Floods can carry much larger particles - cobbles, boulders, houses, etc.
2. Suspended Load (cont.)
Type and amount of material carried is controlled by
Velocity of the water &
Settling velocity of the particle (the speed at which a particle falls through a still fluid)
The slower the settling velocity and the stronger the turbulence, the longer the particle stays in suspension and the further it will be carried.
3. Bed Load
Sediment too large to be carried in suspension - particles that are moved along the bottom of the stream (the bed) by rolling, sliding and saltation.
Capacity
The maximum load of solid particles that a stream can transport -
Discharge determines capacity -
the more water - the greater the capacity
Competence
The maximum particle size that a stream can transport
Velocity determines competence
During floods capacity and competence increase greatly - note the missing bridge.
Alluvium
Term for all stream deposited sediment.
Stream Valleys - 1. Far from Base Level narrow deep V-shaped Valley
2. Meandering River - closer to base level
3. - old well developed stream located near base level - gentle gradient
Floodplains
Alongside stream channels are relatively flat areas known as floodplains. Floodplains develop when streams over-top their levees (sides) spreading water and suspended sediments over the land surface during floods.
Upon retreat of the flood waters, stream velocities are reduced causing deposition. Repeated flood cycles over time can result in the deposition of many successive layers of alluvial material.
Levees
Natural levees flank the stream channel, created by deposition of coarse material.
Backwater Swamps
Marshes created by poor drainage caused by natural levees - water canŐt drain back into river after a flood.
Yazoo Tributary
Tributary streams channels which canŐt join with the main channel because of the natural levees
Runs parallel to main channel for many kilometers until it can break across a levee and join the main channel
Oxbow lakes
The upstream leg of a meander bend catches up with the downstream leg. River erodes though the neck between the legs, cutting off the meander loop. The abandon loop is called an oxbow lake.
Other Stream Forms - Braided Rivers
High sediment load - exceeds capacity of stream - abrupt velocity decrease - creates complex interwoven network of channels
Alluvial Fans
Deposited on land where a high gradient stream leaves narrow valley and suddenly enters a broad flat plain or valley floor.
Deltas
Stream enters a large still body of water, like an ocean or lake.
When a stream approaches a lake or other still body of water, its velocity quickly drops to zero and its load drops out.
Stream Networks
Stream join together in a network
A drainage basin is the land area that contributes water to the streams
Drainage Patterns
The topography and geology of an area control what kind of a drainage pattern develops.