Effects of Bridges and Culverts


Stream channels are shaped during heavy storm events that produce high flows. In a healthy stream channel, the energy of those high flows is dissipated when the stream spills out into the flood plain. But when those high flows are forced through a narrow bridge or culvert opening that restricts access to the flood plain, it creates a "nozzle effect" increasing stream velocity and erosive forces downstream. Scour of the stream bottom and bank erosion are often the result. Because mussels rely heavily on a stable stream channel, these types of habitat alterations can have a drastic localized impact on the fauna.
	The culvert in this picture has caused a large scour pool downstream. Scour like this can lead to the deepening and widening of the stream channel downstream. This leads to a decline in mussel populations downstream.
	A stream channel's susceptibility to scour and erosion varies with the soil geology of that particular location. Our culvert study sites appear on this map, and we found that sites in the eastern region of the study area exhibited a much higher degree of channel alteration downstream. Consequently, mussel populations were also highly impacted downstream.
	Bridges, such as this one, that provide relatively good access to the floodplain cause the least amount of damage to the habitat downstream.