Across British Columbia, roads often cross or run alongside wetlands. When this happens, they can change how water moves across the land and reduce habitat for fish and wildlife.
Wetlands provide year-round food, cover, and water for a diverse range of fish and wildlife species, including moose, elk, waterfowl, salmon, and beavers. They also support frogs, turtles, and insects that form the foundation of the food web. These vital areas help maintain cool, clean water, offer spawning and rearing habitat for fish, and create nesting spaces for waterfowl, all amid lush vegetation.
When roads disrupt natural water flow, streams can erode and deepen, sediment can wash into fish habitat, and wetlands can slowly dry out.
The following illustrations show common ways roads can affect wetlands and the species that depend on them. These examples help land stewards recognize early warning signs and understand how small changes can lead to larger habitat impacts over time. Thank you to Petra Hekkenberg for these illustrations.
1. Road Prism Blocking Natural Flow
The road prism includes the road surface, base, and side slopes. When a road is built across a wetland or stream, it can block or redirect water.
Without a proper crossing, water may pool on the upstream side while downstream areas receive less flow. This can flood vegetation on one side and dry out habitat on the other. Sediment may also collect at the crossing.
When culverts are properly sized and installed, water can move more naturally under the road. This helps reduce flooding, prevent drying downstream, and improve fish passage.
2. Channel Erosion and Headcut Formation
When a road crosses or runs beside a wetland, it can channelize water and make it flow faster. Faster water can cut down into the streambed, creating a steep drop called a headcut. Over time, this drop moves upstream and the channel becomes deeper.
As the channel deepens, banks become steeper and less stable, often leading to erosion and slumping. At the same time, the water table lowers and nearby wetland soils begin to dry out.
Over time, the wetland can shift from a wet, diverse habitat to a drier system with fewer plants and reduced water storage. This reduces habitat quality for fish and wildlife.
3. Wetland Drainage from Roadside Ditches
Roadside ditches are built to move water away from the road. When a ditch runs through or beside a wetland, it can slowly drain water away from the wetland.
As a result, groundwater and surface water move into the ditch, lowering the water level in the wetland. This process is called dewatering.
As water levels drop, wetlands begin to dry out, soils become exposed, wetland species become stressed, and upland plants/weeds move in. Even shallow ditches can cause long-term drying that reduces habitat for fish and wildlife.
4. Sediment and Water Diversion from Road Ditches
When a ditch sits lower than a nearby wetland, it can pull water away that would normally feed the wetland. At the same time, rain can wash soil from roads and slopes into nearby wetlands.
As water is diverted and sediment builds up, wetland channels can fill in and water can become cloudy. Sediment can smother plants, reduce water clarity, and damage fish habitat. Over time, this combination of drainage and soil buildup weakens the wetland and reduces its ability to support wildlife.
BCWF’s Road Impact Wetland Health Assessment (RIWHA) Project
In 2025, we developed and field-tested the Road Intersect Wetland Health Assessment (RIWHA) project. The RIWHA project was created to better understand how roads are affecting wetlands and the fish and wildlife that depend on them.
Across B.C., many wetlands intersect with forestry and rural access roads. Even small design features, such as ditch placement, culvert size, or road height, can change water flow and reduce habitat quality over time.
Throughout the summer of 2025, BCWF field staff worked alongside several First Nations partners in Northern B.C. to collect RIWHA data.
The aim of this work is to document signs of stream erosion, including headcuts, sediment entering wetlands, ditches draining water away, drying soils, and water pooling at road crossings. Additionally, this project seeks to develop a practical, easy-to-use tool that assists land managers in identifying early warning signs and prioritizing restoration efforts before the impacts become severe.
Read more about the RIWHA project here.
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