While strolling along a rather controversial Forest Service road the other day, I came across a culvert that had been deliberately plugged up with rocks and debris. The thinking had evidently been that the water that would have gone through the culvert would now wash over the road and gradually erode it away, making the road impassable to logging trucks and other vermin. While displays of enmity toward roads and the species that find them useful are certainly understandable, the action of plugging culverts also brings about a number of ecologically unpleasant effects that should be seriously considered by any midnight recreationist.
To understand what happens when culverts don't work, one should first consider what happens when they do work. Culverts are installed for two primary reasons: first, because roads cross streams occasionally, and second, because roads generally screw up natural drainage patterns. Culverts at stream crossings act as cheap bridges and let the water flow under the road without picking up sediment at the road-stream intersection. Culverts located mid slope are often called relief culverts, and their job is to restore some natural drainage. These culverts collect water from the inside ditch and pass it under the road and down the hill, where it will theoretically be soaked up by soil and vegetation and separated from the sediments it bears.
When you plug a culvert at a stream crossing, the result is immediate and obvious: the road forms a dam in the stream, and the water goes about the business of eroding the road away bit by bit, cutting its way down to the original stream bed. If things go well, this will be gradual and will only slice out a thin section of the road, and add only a few cubic yards of sediment to the stream (roughly the same amount that you screamed about when the road was built in the first place). If things do not go well, the road bed at the crossing will become saturated and will fall off of the hillside like a big lump of wet cement. If things go really badly, the lump of wet road bed will start a debris torrent in the creek. This will tear out the stream bed all the way down the mountainside and deposit tons of soil, brush, trees and boulders at the mouth of the stream. This is not pretty (the accompanying photo illustrates the result when a culvert happened to be plugged naturally), particularly for the fish attempting to live and spawn in that huge pile of muck you and the road just put in the river below.
When you plug a relief culvert, the results may not be so immediate or obvious, but may be just as catastrophic for fish. The next time there is a big storm, or next spring when there is plenty of snow melt, water will likely run down the inside ditch all the way to the nearest stream, merrily picking up road sediment along the way. Or, water will simply soak into the road bed or run over the road, causing the same sort of mess that can happen at stream crossings.
Short of lobbing dirt clods into the creek, there is no more effective way of adding sediment to a watercourse than by plugging culverts. So, I say, don't do it. Ever.
Removing culverts altogether, though, is an entirely different matter. When culverts are installed, they generally come pretty close to following the natural contour of the land. If you dig one up, what you have left is a ditch that is more or less where the natural drainage occurred in the first place. The trick is making this ditch large enough to accommodate peak stream flows and letting the water find its natural course without adding a bunch of that nasty road sediment to it. Here's how you can do it:
1) Working at low stream flow (usually late summer or fall), dig a trench in the road several feet to the side of the culvert. Skip this step and go to step 3 if you are working on a relief culvert that's already dry this time of year. Line the new trench with some kind of plastic (go to K-Mart and get one of those long plastic sheets you spread out on the lawn and run water down so kids can jump on and go sliding across).
2) Divert the stream into your plastic-lined trench. Plastic sheets draped over something solid work pretty well for a temporary diversion dam.
3) Dig up the culvert, which should now be dry. Don't forget to punch a few good sized holes in it if you are opposed to culvert reuse. [Ed. note-For other creative uses of culverts, see the Jack article on front page.]
4) Enlarge the trench the culvert was in to about five times as big as the most water you could ever imagine coming down the stream. THINK BIG. The culvert itself was generally adequate to handle 10 or 25 year high flows if it was old, or 50 to 100 year high flows if it was new. You want your reclaimed stream bed to last forever.
5) Try to get as much fine soil out of the culvert trench as you can. It's best if you can get it down to bare rock, or fill the trench back in a bit with cobble-sized rocks.
6) When you are happy that the old culvert trench is BIG and the bottom is made of rock, not dirt, divert the stream back from your slip 'n' slide.
7) Install a nice sign reminding drivers that the road is closed.
8) Go to work forcing the Forest Service to do all this hard work so you don't have to, and remind them to return the entire road to contour, not just pull the culverts.