Bear Lake Methods: Sediment Transport Physics

Sediment transport physics:

Large particles are usually heavier than small ones, and take more energy to move around. A very strong wind is needed to move sand-sized grains while a trickle of water moves them easily. Since it takes more energy to move water than air, the moving water already contained more energy than the wind. The energy added to the water to put it in motion is transferred to the sand, to put the sand in motion. As soon as the water stops moving, it loses the energy needed to keep the sand in suspension. At the mouth of rivers, where they enter lakes and oceans, deltas form from sediment the water no longer had energy to carry.

River energy is supplied by gravity, as the water flows from high elevations to lower elevations. The steeper the elevational gradient, the faster the water moves and the more energy it carries. More water also carries more energy, so a flooding stream can transport more sediment than the same stream at normal water levels. Depending on their energy, rivers transport a wide range of particles as their sediment load, from clay-sized glacial flour to large granite boulders.

Lakes have energy supplied by wind and density currents. Wind creates waves that stir up sediment in the shallows and move it, usually towards and away from shore. Wind also generates surface currents that move sediment along shore. Like rivers, density currents are driven by gravity and can be thought of as internal rivers. Just as rivers flow down elevational gradients, so lake water flows down density gradients creating a current as it moves. Density gradients are formed by differences in temperature, sediment load, or dissolved mineral load.

Cold water is denser than warm water (because the molecules are packed closer together), so it tends to sink. Likewise mineral-rich water sinks beneath fresh water. Often wind will help create density currents by moving water to the surface where air temperatures chill or heat it. Mineral springs entering a lake create density currents as heavier, mineralized water sinks through the fresher water of the lake. Hot mineral springs will not sink as far, because the weight of the minerals is offset somewhat by the lower density of the warm water. In the Great Salt Lake, the opposite is more likely, where relatively fresh springs upwelling in the salty lake rise above the denser water around it.

For more information on currents in lakes and oceans, try these links:
Water on the web - understanding lake ecology
A Primer on Ocean Currents
Circulation of the ocean waters - Britannica.com
Sediment Transport of an Estuary Estuarine Ecology

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