As mentioned previously, rivers deposit much of their sediment when they enter an impoundment. Any contamination deposited with the sediments is trapped as well. For example, during the planning process for the removal of the Oak Street Dam in Baraboo, coal tar was found in the sediments in part of the impoundment. The coal tar was stabilized and removed before the dam was removed to prevent contamination of the river downstream. Sediment cores can be taken if there is any contamination suspected.
Nutrients tend to build up within an impounded system more quickly than in free-flowing systems.
High nutrient levels in impoundments can lead to eutrophication of the impoundment. This means that the water contains enough nutrients to support nuisance growths of aquatic plants and algae blooms. When these algae blooms die, the decay process uses up oxygen dissolved in the water, reducing the amount of oxygen available to fish and other aquatic life. The turbidity caused by decaying algae and suspended sediment can cause problems for other plants. Plants rely on sunlight reaching them to carry out photosynthesis.