Removal of large rocks from a stream can decrease the stream's resistance to flow. This results in an increase in the stream's velocity. Increased velocity can cause downstream flooding. In addition, the removal of large rocks often leaves a stream bed with small, uniform-size rocks. This destroys important habitats of aquatic plants, fish, amphibians, and aquatic insects. Many of these organisms require areas with large rocks for cover, oxygenated water, feeding, and even for the placement of eggs (see photo below). Photo: Larvae on a rock from Wheeling Creek. Photo courtesy of Dr. Kenneth Rastall.
Dredging is generally performed with large machinery. Large machines in small streams disrupt the stream bed and cause a short-term siltation event. The siltation event can be harmful to downstream aquatic communities. Siltation is observed as increased turbidity . Increased turbidity can lead to increased water temperature.
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