Culverts are large pipes that pass water under roads to protect them from erosion or flooding.  Culverts are especially popular for small streams or in remote areas where building a bridge would be too expensive or impractical.  Though most culverts do a pretty good job of passing water under the road, they often do not allow fish to pass.  The downstream end of the culvert may be too far above the waters surface for upstream migrating fish to enter.   Water in the culvert may be moving too swiftly, or be too shallow for fish to pass in either direction.  Debris may also collect in the culvert, not only blocking fish passage, but water as well. During floods, blocked culverts are responsible for many road failures.

Because there are so many of the, culverts may be a very significant barrier to fish habitat in a watershed.  Culvert improvements typically involve lowering the downstream end of the culvert to the water’s surface, reducing the steepness of the culvert, and making design modifications to reduce water velocity and increase water depth in the culvert.