Stream Crossings

Where a road crosses a natural watercourse, provision must be made to carry the water under or across the road. The selection of the best and most appropriate stream crossing design depends on a number of factors, and a poor design choice can result in a costly installation that is subject to failure and may cause significant environmental damage.


Common features of stream crossings that lead to erosion problems include:

  • Filled crossings without culverts.
  • Crossings with undersized culverts.
  • Crossings with culverts susceptible to being plugged.
  • Crossings with culvert outlet erosion.
  • Crossings with a potential for stream diversion.
  • Crossings that have diverted streams.
Two people in a hilly, muddy area with a large pipe; one person is in a yellow jacket. Green vegetation and rocks are visible.
This stream crossing washed out (eroded) when the culvert plugged and streamflow overtopped and scoured through the road fill. Sediment was delivered directly to the stream crossing as the fill eroded. Stream crossings and culverts need to be properly designed, constructed, and maintained to minimize the potential for such failures. (PWA)

Examples of properly designed stream crossings

Culvert re-sizing:

A wooded area featuring a small creek, fallen logs, and a large pipe structure, surrounded by lush greenery and rocks.
Before: Undersized, perched culvert on a fish bearing stream creating a fish passage barrier.
A metal culvert tunnel under a rocky landscape with a stream running through it, surrounded by trees and a fenced area.
After: Properly sized culvert to pass the 100-year design flow on a fish bearing stream that is installed inline with the natural channel gradient and natural gradient. The culvert is embedded into the streambed to allow fish passage at all stages. The greater than 2:1 slope outboard fillslope is armored with riprap.

Bridge installation:

An opening of a dark pipe surrounded by rocks and foliage in a forested area. Sunlight filtering through the trees.
Before: Undersized culvert crossing with steep eroding fillslopes.
A wooden bridge spans a rocky ravine in a wooded area, surrounded by trees and fallen leaves.
After: During a road to trail conversion, this stream crossing was upgraded to a bridge crossing with the road fill removed and natural channel gradient restored.

Armored Fill:

A blue SUV parked near a dirt road with a person walking nearby, surrounded by trees and dry foliage.
Before: Undersized, perched culvert on an intermittent stream prone to inlet plugging and outlet erosion. 
A pickup truck parked near a stream in a wooded area, surrounded by trees and greenery.
After: Armored fill crossing with enough capacity to pass the 100-year design flow without diverting. Some fill is left in the crossing so vehicles will have a level roadbed to cross the channel.

Trash Rack:

A metal drainage pipe with water flowing through it, surrounded by rocks, in a wooded area. An ATV can be seen in the background.
A single post-trash rack installed slightly upstream of the culvert inlet to reduce plug potential.

Critical Dip:

The image shows a road, a properly-sized culvert, a critical dip, and a single-post trash rack in a natural setting.
Culvert crossing installed at the base of fill with enough capacity to pass the 100-year design flow. In addition, a trash rack has been installed to prevent culvert plugging, and a critical dip has been added on the down-road hinge line of the fill, preventing stream diversion.

Stream Crossing Decomissioning

Removing (decommissioning) a stream crossing involves excavating and removing all fill materials placed in the stream channel when the crossing was built. Fill material should be excavated to recreate the original channel grade (slope) and orientation, with a channel bed that is as wide, or slightly wider, than the original watercourse. If the channel sideslopes were disturbed, they should be graded (excavated) back to a stable angle (generally less than 50% (2:1)) to prevent slumping and soil movement. The bare soils should then be mulched, seeded and planted to minimize erosion until vegetation can protect the surface, and the approaching road segments should be cross-road drained to prevent road runoff from discharging across the freshly excavated channel sideslopes.

Here is an example of decomissioning a stream crossing:

A forested area with two dirt paths, a pickup truck, and people assessing the surroundings amidst cut trees and debris.
An excavator works on a dirt site in a forested area, with a winding road visible in the background.
Once cleared, an excavator excavated the fill material and several dump trucks were used to endhaul spoil material to a nearby, stable disposal site.
The image shows a forest area with tree stumps, fallen logs, and a dry streambed, indicating possible logging or land clearing.
The channel was excavated (exhumed), and the sideslopes were sloped back to stable angles, exposing several buried stumps that signaled the level of the original ground surface. The bare slopes were then seeded, mulched with straw, and planted with trees. All the logs visible on the right bank had been buried in the crossing fill. They were removed and placed on the final ground surface.
The image shows a forested area with tree stumps, greenery, and a dry creek bed, indicating past logging or disturbance.

Four years later, the site showed significant revegetation, and minimal channel adjustment (scour) or surface erosion.

What should I do about stream crossing problems?

  • Contact the Marin RCD: call, email, or use the submission form in the sidebar of this page.
  • Secure permits: This practice always requires permits.