Jump to main content or area navigation.

Contact Us

CADDIS Volume 2: Sources, Stressors & Responses

energy sources physical habitat hydrology temperature water and sediment quality stormwater runoff wastewater inputs riparian and channel alteration urbanization

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).

Click below for more information on specific topics

channel enlargement button road crossings button bed substrates and biotic condition button
Example culvert (top) and bridge (bottom) road crossings.
Courtesy of Tetra Tech.
Figure 42. Number of adult caddisflies caught directly upstream of bridges and culverts (n = 8), vs. at control sites 50 m downstream.
From Blakely TJ et al. 2006. Barriers to the recovery of aquatic insect communities in urban streams. Freshwater Biology 51:1634-1645. Reprinted with permission.

Jump to main content.