Tree revetment is a technique of laying trees along a stream side in a manner that reduces the erosive effects of the stream, protects the stream bank, and provides instant habitat for small fish. In 1996 about 350 cedar and pine trees were anchored to the bank and strategically placed along a 400 foot stretch on our riverbank.
To prevent the river from going behind and undercutting the revetments, large rocks (rip rap) were placed along the groomed stream bank both at the upstream and downstream ends of the structures.
Most of the stream bank protective work done on Habitat Farm was revetment work, but other methods were also employed. Tree matting was used on one outside curve of the river. Tree matting is similar to revetment except that smaller trees are piled on each other along a groomed stream bank, then anchored with rocks on top. They again create an energy absorbing friction and instant habitat.
The combination of all three methods were required because of the nature of the stream. The result is a complex that is protecting our stream bank in a manner that tries to approximate nature without compromising other important considerations. The first try wasn't 100% successful. Because there were high voltage power transmission lines on one of the river curves, the power company insisted on a high degree of erosion prevention. On that corner only rip-rap was used. Rip-rap, we learned, does little to absorb the river's energy. It only transfers it. Analogous to banking a billiard ball, the energy of the river simply bounced off the rip-rapped corner and caused an increased level of erosion at the next weakest spot, the next downstream corner; the neighbor's place.
The following year, Fisheries returned, and using large boulders constructed a weir at the downstream toe of the rip-rapped corner to change the direction of the flow more to the middle of the river. This succeeded in transferring the river's energy downwards rather than across the river. It caused the scouring of a deep hole below the weir. If you can imagine whipping a rope up and down and seeing the wave travel down it's length, the shift of flow caused by the weir caused the flow energy to come back up from that hole and about another 50 yards downstream scour out another. We thought these were excellent examples of the ripple effects, almost, of any changes that occur in a stream. Like the way a wave travels the length of a whipped rope, in stream changes have downstream consequences until the energy of the change is ultimately absorbed by other influences.
Using revetment, or some other method of using trees strategically placed on the stream bank tends to create the necessary energy absorbing friction needed to reduce the transfer of energy downstream. Plus it creates instant protective in stream habitat for young fish. The problem with woody material in the stream, however, is that it wears out. The projection is that the revetment on Habitat Farm, where it continues to be exposed to the river during freshet, will start to break down after five years. That's one important reason for replanting the stream bank with live trees immediately after the revetments are put it. The hope is that the root mass of new trees will replace the worn out revetment wood. Of course the other important reason for planting trees is for creating the necessary shade to help keep more water, and keep it cooler and cleaner. We installed setback fencing the entire length of the restoration project to protect the newly planted vegetation from our sheep.
During one part of one June day a few years ago, the river eroded the bank in the photo to become a wide shallow piece of stream. We were left with what you see between the riverbanks. Nothing but gravel. We lost about 3 acres that year. In the three years since doing the restoration work here in the photo and for a few hundred yards upstream on our property and the properties of two upstream neighbors, you can see how the river has began to resettle and the stream sides have started to reestablish.