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 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 movement 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 catchment. 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 availability, 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 potentials, and higher concentrations of ammonium, manganese, iron, and hydrogen sulfide.
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