Deep Impoundments

In addition to the distinctions described above, deep impoundments have some specific attributes.

Gradients and directionality

A one directional longitudinal gradient with maximum depth at the downstream end is typical of impoundments created by damming a river. In lakes, the deepest region is typically more central, with the major flows being rotational, driven mainly by wind. The continuous

Figure 1 Ford Lake dam and power station, southeastern Michigan, USA, forming one of hundreds of impoundments constructed in the early twentieth century to supply electrical power to industry.

declination of the impoundment bottom from the inflow to the dam is usually accompanied by broadening at the inflow end. The result is a mostly unidirectional move ment of water from inflow to outflow.

Longitudinal gradients in many properties are seen along the length of an impoundment. The inflow region receives high concentrations of particles, organic matter, nutrients, and other chemical components from the upstream catch ment. Localized flushing rates are faster and biological activity is lower than in more slowly flushed, larger volume elements down the impoundment. In the transition zone as flow is slackened, primary production is increased toward the capacity provided by nutrients, underwater light avail ability, and predation pressure of zooplankton. Increased phytoplankton chlorophyll a concentrations, coupled with sinking and decomposition of organic matter result in lower oxygen concentrations near the bottom, low redox poten tials, and higher concentrations of ammonium, manganese, iron, and hydrogen sulfide.

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