Question
I Would like to put small river rock down where the rain spout drains. I have dug down abut 6". What should i put down before the rocks. Sand? weed control paper? plastic? I had mulch before but as you know it washes out. Their will be mulch next to the rock.  

Answer
Ron:
I would dig out an area about 24" deep and 24" wide by 24" long and line it with filter fabric (weed mat will work too as long as it allows water through). Then fill it with cheap gravel. Leave the cheap gravel a few inches shy of reaching the top. Then cover the gravel with more filter fabric. Now cover the fabric with 2-3" of the decorative rock of your choice. "Wa-la" Now you have an old fashioned "dry-well" which will help with erosion and retain water during short rain falls allowing it to percolate into the soil. For best results place this several feet or more away from your house and pipe the down spout into the gravel 'dry-well-. If your house is large or the gutter serves a large part of the roof make the 'dry-well' bigger and deeper. You may do this at several locations around the house for best results. Beware, if your house has a basement or crawl space, adding water to the soil may cause problems if your waterproofing is not up to standards.

However you can do this, it is a great example of a Best Management Practices (BMP's) and is great for our environment and your yard.
Best of Luck

Sean J Murphy, LA,ISA, LEED AP
Please check out my blogs and articles on landscape topics like this on my websites.
http://www.seanjmurphy.com
http://amenityarchitects.com