The side of my house has a small gravel bed that abuts the lawn. It is mostly choked with weeds and was in poor shape when we bought the place last year.
I want to dig it up, put down new fabric and reuse the existing river rock that is there. I don't want to reuse the existing weeds...
Any advice on how to clean the old gravel out so that I don't have weeds next spring?
It sounds like you are on the right track. Depending on how "small" the gravel bed is, you could dig the gravel into a wheel barrow or trough, and then fill the container with water to float the weeds and "clean" the gravel.
Then you'd have to deal with the weed seed bank that has surely developed in the soil under the gravel. Landscape fabric might do it, but you could also just use cardboard as a base layer. The cardboard should let enough water through to prevent ponding, will block light, and will decompose after a while - and you won't have to dispose of landscaping fabric again.
Going forward you'll want to deal with the weeds regularly before they have a chance to go to seed. If the gravel makes hand removal difficult, boiling water is a quick way to kill weeds in this situation - if you stay on top of them. Once the weeds have browned after boiling water application, you should be able to just rake the dead weeds out.
For removal of weeds from a small area, the boiling water method works like a charm and I have been using that method for years with great success. I hate using weed killers because of the damage it does to the environment and I have plenty of beautiful frogs around my place to prove that it does work.
You could try making all the weed seeds germinate this year. It's late enough so that unless they're perennials or cold-tolerant annuals (or have a very short life-cycle), they should die with the cold without reseeding again (after they germinate this year, that is). If enough of them sprout this year, you may not have a problem next year (that's the hope, anyway). I don't know what kind of weeds you're dealing with; so, this may not be a viable solution.
To get them to sprout, you could water it pretty well for about a week. That by itself should get a lot of them to sprout, but if you want higher germination rates to ensure you don't miss as many for next year, you could add some food grade hydrogen peroxide (not the regular kind, since it can contain harmful stabilizers such as organophosphates and/or heavy metals; you can contact the manufacturer to see what they add) to the water or electrify the ground (both of which are said to help bring seeds out of dormancy, whether or not it's true); I'm not sure how effective they are at doing this, however. If you use too much peroxide (and/or electricity) and kill the seeds, I guess that won't hurt matters. The hydrogen peroxide should turn into water eventually. It would probably be safe to use the extra-potent food grade kind, if you wanted to.
Anyway, I don't know if this solution will help at all. It's just an untried random thought (granted, I'm in the process of trying the water only method, but as we're having an awesomely late frost this year, maybe I better just pull the weeds up, and that might be a good idea anyway, since they might be perennials).
If you want to be able to use that space I would reconsider re-using river rock. Tough to put a table or chairs on and certainly hard to walk upon. Much better use of your time would be to dispose of the river rock somewhere on your property that doesn't get much traffic. Or ask your friends or neighbors if they'd like 'free' river rock. They just have to remove all the river rock from your property.
To prepare for new gravel make sure you are at least 4" below grade. Then cover the area with landscape fabric. You WANT it to stay in place a long time. This is the ONLY purpose of landscape fabric. It is not for weed protection! Landscape fabric will prevent your gravel from disappearing as weight is applied to the surface of the gravel. Without landscape fabric, gravel will sink down into the soil and the fine soil will come to the surface. Then you will have a mess on your hands!
The gravel you should get is called 3/4 minus or even 3/8 minus. They regrind the normal utilitarian 5/8 minus crushed gravel you've seen in driveways. The smaller size looks more refined, professional. To figure out how much to order, take your square footage of the area (do all of this with any pathways you'd like to have to take you to your new informal 'patio' as well) and divide by 81. This gives you cubic yards to order or pick up.
Make sure you edge the areas to be graveled with pressure treated 2X4 and stakes. Treks, dimensional lumber made from recycled plastic bottles is a great product for this. Don't use any other color than dove gray! Keep the top of the 2X4's at grade, not above. Fill with 4" crushed gravel with the fines (the 'minus' part of the 3/8 minus term). Rent a compactor and compact your 'patio' and walkways. This provides a comfortable, clean surface that you can walk upon, place pots, furniture and stays in place. VERY tough for weed seeds to germinate. If they do either pull them when they are tiny or use glyphosate (Round-up). Killing weeds in gravel is the only time I even use this stuff. The weeds should be young and growing vigorously before you spray. Be careful with over-spray or spraying too fine a mist on a windy day!! You probably won't ever need to use it if you use 4" of compacted gravel over landscape fabric!
Whatever you do, don't use any color in the gravel or wood. It is distracting in a garden. Keep all your hardscapes a dove gray and your work will look professional. That is my opinion as a Landscape Architect! Grin! Hope this meets your needs.
Similar to Allen's boiling water method, you could try solarizing the patch, if it gets enough sun. Basically, you'd just lay a clear plastic tarp over the area and roast the seeds to death. It takes a few weeks during hot, sunny weather, but I think this is the least labor-intensive method suggested so far.
http://www.organicgardening.com/learn-and-grow/soil-solarization
