How Streams Cross Roads
Can't Go Over, Can't Go Around, Gotta Go Through It


Pennsylvania is home to the second largest number of stream miles in the U.S. with 86,000 miles. It is also home to 41,000 miles of roads that often cross these streams around the state. Streams are important habitats for all sorts of aquatic organisms in our ecosystems, and when crossings are constructed without consideration for the role they play in maintaining streams, they can cause harm to the aquatic environment.

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How Streams Cross Roads
Stream crossings, sometimes called culverts or crosspipes, connect one side of the stream to another while allowing traffic to pass above. In order for these crossings to function for the stream as well as the cars, it is essential that the stream flows uninterrupted through the manufactured addition.

Importance of Stream Bed Material
One way that stream crossings can be installed to support stream health is by connecting both sides of the waterway with stream bed material through the pipe. Stream bed material inside culverts is essential in ensuring the flow of the stream mimics that of the natural ecosystem. This allows the passage of aquatic organisms like crayfish or trout.

There are other benefits to maintaining a stream environment. Installing stream bed material inside a culvert also helps dissipate energy, which prevents the channel from picking up too much speed through the structure causing erosion downstream. Benefits extend beyond environmental impacts, such as protection of the pipe to reduce deterioration of the structure over time.

Re-Creating a Stream Environment
When installing stream bed material inside of a crossing, it is important to re-create the stream environment. This means including a “low flow channel” where the stream material is lower in the center of the pipe and built up on the sides. These low flow channels keep water and natural sediment moving through the pipe and ensure the best aquatic organism passage (AOP). Establishing riffles and pools – or high points and low points – in the structure can also be a critical piece of maintaining the stream ecosystem inside a culvert.
A Stream Crossing in Action
Placing stream bed material inside a culvert can be tricky depending on the size of the structure. Recently, staff from Allegheny County Conservation District and the Penn State Center for Dirt and Gravel Roads assisted a local Low Volume Road Grant recipient in installing material in a stream crossing in Braddock Hills.

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After placing larger stones inside the pipe to create raised bank margins and a low flow channel in the middle (top photo) the team sought assistance from a local fire department to place smaller material in the cracks and crevices of the newly created channel (bottom photo).

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Smaller material keeps the flow of the stream on top of the rocks instead of washing along the bottom of the pipe. Using a fire hose, the team pushed the smaller material through the pipe and recreated the channel. Pressure from the fire hose also ensures good compaction to prevent material from washing out of the pipe during a heavy storm.

The Dirt Gravel and Low Volume Roads Program funds projects on roads around Allegheny County with a goal of both improving road conditions and the environment.