Question
I'm installing a river rock creekbed as a landscape feature, but that will also channel water during heavy rains (which we have plenty of in W. Washington!).  the soil is a weird mix around here, but includes plenty of silty material that I'd rather not have flowing out the lower end the the creek.  Is it advisable to lay sand and/or gravel under the river rock?

Answer
Hello,

Do not place sand or gravel under the rock. The water will simply wash it back out. Sand and gravel placed under the rocks will do nothing to aid drainage with such a large volume of water moving at speed. Since the sand or gravel will likely be undermined,it will not make a good storage area for sediment and even if it were to remain stable,it would eventually completely clog with sediment and be rendered ineffective. Cleaning it out would be inconvenient,since it would be covered with rock and mixed with heavy sand and gravel.

If you would like to avoid this maintenance it may be possible to simply "let ole mother nature have her way" and let the sediment build up naturally. Such a situation would be what occurs in a natural stream bed. The water will change course as the sediment deposits,creating an "ox bow" effect and the sediment will fan out.

You should try to capture sediment at the entrance to the creek. This will keep sediment out of the creek and make cleaning it out more convenient. This can be accomplished by constructing a pool area with check dams or plants,if necessary,to slow down the water. The pool should be higher in elevation than the creek and have a spillway for controlling entry of water into the creek. Line the entry point to the pool with rocks and/or plants to prevent erosion and plant the entire pool area to further trap sediment. The outlet can be treated in a similar fashion if erosion will be a problem there. There are devices to control overflow at the outlet. Good luck.

Mark Harshman


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