Effects of road crossings

Roads can adversely affect stream ecosystems via multiple pathways. Indirect effects include:

• Altered stream discharge patterns due to increased imperviousness and stormwater runoff
• Increased contaminant loads due to accumulation on and runoff from road surfaces

At road crossings, roads can directly impact stream ecosystems, for example by altering channel geomorphology, increasing sedimentation, and impeding fish and invertebrate movement. In addition, stormwater drains often run along roads, and road crossings frequently are points of stormwater discharge to streams. Thus, road crossing density can be a good predictor of stream biotic integrity, with biotic condition decreasing as the number of road crossings increases (Alberti et al. 2007, Carlisle et al. 2009).

However, not all road crossing types have the same effect. For example, Blakely et al. (2006) examined how different road crossing types affected movement of adult caddisflies in New Zealand streams. They found that road culverts were barriers to caddisfly dispersal: the number of adults caught immediately upstream of culvert crossings was much lower than the number caught at control sites downstream (Fig 42). Bridges, which provided more open spans over streams, did not inhibit movement. Fish movement has shown similar bridge vs. culvert patterns (Benton et al. 2008).

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