Water flows, as it will, taking the easiest path, always going downhill. Obstructions are worked around, or under, slowly washing away bit by bit. People who live near running water always watch to make sure that they are not part of that which is washed away.
One of my friends is a waterway fixer, repairing hillsides that have washed down, slowing streams that have suddenly begun eroding through their beds, and reshaping river banks with stone, logs and new plantings. He was in town a couple of weeks ago to give a workshop for the Mendenhall Watershed Partnership, one focused on hands-on techniques for slope stabilization.
We worked on a slope in the Montana Creek Drainage, first laying bundled willow branches in eroding surface gullies and partially covering them with earth from the sides of the gullies, staking the bundles into place. The willow will root and sprout in the wet gullies and help stabilize the hillside.
The entire hillside can also be revegetated and stabilized using similar techniques, and we were shown how to do so. We cut open the bottom of the hillside, where it would pour onto the road as it slipped down, and laid a large log as a base. The log is lightly buried and the soil layer above it sloped back into the hill. We had cut large quantities of willow branches and the whole group set to work trimming off the side branches and cutting them into three- or four-foot-long pieces, which we laid thickly into the sloped trench.
These branches were one to three inches in diameter, and the end closest to the top of the tree was always pointed out of the trench. Bottoms always grow roots and tops always grow leaves, mixing them up is frustrating and a waste of time. Each foot of trench held six or eight branches. So thickly were they laid that it looked like a palisade, and as the soil above was pulled down on top of the layer, it left a ridge of stick tops poking out.
We buried these branches, sloping them back and down into the soil, and another layer was laid on top of them about a foot or two above the first set. This technique continued right up the hillside, with the expectation that they would all root and begin the knitting together of the slope.
This ancient technique, called brush layering, can be used in almost any moist site, and on any scale. German riverside stabilization efforts have been used on ten-mile long sections with great results, and all over North America similar projects are being undertaken. Combining vegetative stabilization within stream work to change the flow patterns of the rivers has stopped the bank-collapsing sequence in hundreds of miles of erosion prone areas.
It is not only rivers and large-scale projects that benefit from brush layering or other similar low-tech work: Smaller sites are the same. We went to a small stream in Mountainside Estates that has begun cutting its way deeper and deeper, the result has been collapse of the sides and rapid loss of the surrounding yards. The families that live along the creek were worried about how far the process would go before it stabilized, or if it ever would.
We determined that the creek had been overdeepened at a culvert crossing, and the new level caused the water to run faster, cutting the rest of the creek bed down to match the bottom of the hole. This process could be seen in a small waterfall that was slowly moving up the stream, washing away the creekbed as it migrated upstream. The little waterfall is called a headcut, and is an indicator of streambed disturbance.
Left untreated the headcut will climb up the stream, lowering the bed to bring it into equilibrium with the dug-out area and changing the profile of the banks as they collapse and are washed away. This treatment is fairly simple to understand, and can be done in stages so that expense and disturbance can be minimized.
The cause of the disturbance was the hole dug into the streambed - so fill it in. Use a mixture of sizes of rock. Smaller ones will fill in the spaces between bigger ones and finer particles will settle into the tiny voids creating a new, higher- profile streambed.
The neighbors will be working on this project, and I will let you know what happens.
David Lendrum is a master gardener and owner of Landscape Alaska. Responses or questions can be sent to www.landscapealaska.com.
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