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In this part of the country, what has the largest impact on salmon? Clear-cut forests? Roads? Fishing? On most of our North Coast streams, sediment is the biggest problem for salmon ? it buries eggs, scours out habitat cover, and changes the very shape of rivers. And most of that sediment comes from roads. Here in the Mattole, researchers estimate that over 80% of sediment generated by human activity in the watershed is directly related to roads.
You may be asking: ?Roads? How do roads so thoroughly screw up a river?? In this series, we will take a look at road design problems, and show how these can be corrected to benefit our local rivers, creeks and fisheries (not to mention ensure access to your home!).
In steep country like this, roads are often built on hillslopes or along creek beds. During construction, a bulldozer operator will blade out a portion of the hillslope, and place the material on perch next to the bladed out area. Roughly half of the road is dug into the hill, and the material excavated in this operation is placed on the slope to create a full roadbed. In this way, the road is created.
Most roads follow a hillslope contour, but all streams go straight downhill. This creates plenty of spots where the road must cross streams. At these stream crossings the road builders have several options. A bridge can be built over the stream. The stream can be routed through a culvert (a metal pipe) under the road. Or the stream can go across the road surface, and drivers will have to fiord the creek. Of these, the bridge is the best solution, because the creek can run freely under it. This is also the most expensive option. So, most roads, especially logging roads, have culverts.
Unfortunately, culverts cause many problems. Since culverts are expensive, there is a temptation to use the smallest culvert possible. The amount of water a culvert can transport is limited by its size. The size can be then further reduced if debris plugs the culvert.
Unlike a bridge, a culvert is not a road surface. Fill must be placed on top of the culvert to make a road surface. If the creek is in a deep gorge, a large amount of fill must be placed above the culvert. On one road in the King Range, one stream crossing was estimated to have 60,000 cubic yards of fill above the culvert!
If the culvert gets backed up, water pools behind it and the road fill functions as a dam. Since it is not engineered as a dam, it often fails quickly. At that point, what was known as ?fill? is now called ?sediment.? This is one way that roads cause problems for fish habitat.
A second set of problems has to do with water on road surfaces, and how it gets off of those road surfaces. The standard solution to road drainage is to shape the road so that water drains off into a ditch on the uphill side, and is then drained downhill through a ?ditch-relief culvert.? These culverts have the same problems that any other culvert does, so they are not an ideal solution, but we will discuss this in more detail in the next issue.
Water has a hard time leaving the road surface gracefully. If water running down a road finds a place to run off of the road, oftentimes that spot will develop a gully rather quickly, as the water has a lot of erosive force as it falls off the steep side of the road.
From reading all of this, you may get the idea that roads are bad, and that interactions between roads and water are the main reason. But let?s face it ? we all use roads to get from one place to another. These problems are not intrinsic to roads, but rather are design problems that can be addressed with some forethought. In the next several issues, we will take a look at how to deal with stream crossings and road drainage, and then will talk a bit about how to survey your roads for potential problems.