form the drop structure. Another less common type of drop structure, the sculpted sloping boulder drop, is derived from this. The sculpted sloping boulder drop is used to create a more natural appearance to the drop structure. Both of these structures also tend to suffer from downstream erosion. Baffle chute The baffle chute drop is built entirely of concrete and is effective with low maintenance needs. They typically consist of a concrete chute lined with "baffle" teeth to slow velocity of water as it passes over the structure. Despite these appeals, however, they have very "limited structural and aesthetic flexibility, which can cause them to be undesirable in most urban settings." Environmental effects Wildlife Drop structures have been shown to either be beneficial or detrimental to habitat in the stream. They create complexity of habitat by breaking up a stretch of stream into a series of pools. Surface turbulence, eddies, and bubbles are generated by drop structures that provide hiding and cover for fish and other aquatic organisms. Water is aerated as it passes over drop structures. Sediment is collected and sorted in scour pools, which provide energy dissipation. On the other hand, drop structures may also become barriers to fish. The downstream channel may erode and slowly and unexpectedly increase the height of the structure, to a point where migratory fish, such as salmon, cannot pass over the structure. Other causes may be that the plunge pool is obstructed or the water flow is too shallow. However, many properly functioning drop structures themselves may impede the upstream and downstream migration of fish. Unless the structure is designed to maintain them, existing fish spawning pools will be impacted or lost. Erosion control Erosion is usually reduced by drop structures, and natural river channel processes, such as channel migration, meandering, and creation of stream pools and riffles, are also reduced. Drop structures can be used for flow control and to stabilize waterways and prevent the formation of gullies. They also have the potential to operate as inlets and outlets for other conservation structures, such as culverts. See also Aquatic sill Check dam Coastal management, to prevent coastal erosion and creation of beach Knickpoint Sill (geology) Weir References Dams by type Stormwater management Soil erosion