87 Of the several types of barriers placed along the river, low-head dams have proven to be the most dangerous and destructive, owing mostly to their design.2 Low-head dams differ from others in that they extend entirely across the width of the river. They control water flow by partially obstructing the channel and creating a backlog of water behind the dam. The water flows over the concrete abutment, ranging from 6 to 25 feet deep. This method of construction allows the current to stream continuously from bank to bank at a safe level. Though initially useful in regulating water levels, low-heads have several adverse environmental effects. The most evident issue these dams present is water quality degradation. This is primarily due to the dam's ability to slow the river’s current, enabling sediments to accumulate, burying fish food, and covering the habitat.3 Slowed currents also lessen the quantity of oxygen in the water, which is essential for fish and insect life. Furthermore, nutrients like phosphorus can accumulate and trigger large algal blooms.4 The production of these blooms is unsightly, smelly and may have a rash effect on wildlife, as it can produce toxins poisonous to fish and humans, resulting in disease or death in severe cases. In addition to quality hazards, dams also have a lasting impact on the ecology of the river system.5 Since they hinder the movement of fish and other aquatic species upstream, they greatly disrupt organisms’ habits and reproductive cycles. Thus, areas of the river with dams have substantially lower biotic integrity and biodiversity than sections without.6 2 Ibid. 3 Cosier, Susan. 2019. “After a Century of Being Dammed Up, Illinois Rivers Get to Go with the Flow.” NRDC. https://www.nrdc.org/stories/after-century-being-dammed-illinois-rivers-get-go-flow. 4 Department of Natural Resources, Department of Natural Resources. 2020. “Safety at Dams - Water Resources.” Illinois.gov. 5 “Low Head Dams/Sills.” n.d. Pearl Riverkeeper. Accessed November 11, 2022. https://www.pearlriverkeeper.com/lowhead-damssills.html. 6 Santucci, Victor, and Stephen Gephard. 2011. “Effects of Multiple Low-Head Dams on Fish, Macroinvertebrates, Habitat, and Water Quality in the Fox River, Illinois.” North American Journal of Fisheries Management .25, no. 3 (January): 975-992.
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