weed fabric does work, the problem is if you use cheap stuff it never works. and even the best stuff only prevents weeds from coming up from teh ground, but does nothing about the ones that sprout in teh debris lodged between teh rocks. the best advice i can give would be to prepare to weed the rocks periodically no matter what you do.

In my opinion this drainage ditch just has to look neat not fancy. While patio pavers or stone looks good there are a lot of cracks for weeds to grow and believe me they will grow. I would lay a thick layer of wet newspaper or plain brown cardboard on the bottom to get rid of any weeds and grass that is there now. then put down a sand-gravel base and lay sidewalk slabs on top. Then you can edge the edges a couple of times a year and a bent nail on a stick run in the cracks occasionally will get the ones between the slabs.

In my opinion landscape cloth of any kind is a waste of money.

Sounds as though you have have bought yourself into a major maintenance problem. Not unusual as we see this type of question everday throughout GardenWeb. It's the old..."I did not pay enough attention to possible problems in the landscape enough when buying this property and now I expect you guys to come up with the easy, cheap, organic (yup, organic) methods to solve what I have overlooked." This is a subject which I have wanted to discuss for a long time on this Forum. I will leave it to others to add thoughts on the subject of potential, costly land problems when buying a home or land.

Do a search for "flame weeding". Done correctly it is the cheapest, easiest organic method I know of to control weedy ditches.

Why not try a bog garden or rain garden approach and make the "weeds" disappear that way.

Alternatively, if the cobbles are large enough and deep enough the sediment accumulation may not be severe enough to allow for weed growth, as the upper layers of rock will shade out the sediment pockets. But this approach will look like shoreline rip-rap. Not foolproof to stop weeds. But you can't do anything that impedes the designed volumetric flow capacity of the drainage structure.

Is it really a 12-foot wide ditch?

I second the idea of using a torch to weed.

How often do you have water in the ditch? How fast does the water flow?

Those ditches sound quite large. You must get heavy rain from time to time where you live. Personally I don't like landscape fabric or rocks as mulch, and would be partial to a native wetland garden instead. But rocks may work well for your particular landscape.

There is a new school that was recently built in our town, and I really like the swales that were created to absorb water around the parking lots, playground, and tennis courts. They were planted with plugs of native wetland flowers and grasses, then mulched with wood chips. I can't wait to see what these swales look like when they grow in and when the wildflowers are blooming.

Here is an picture of one of the smaller swales, I took this about a month ago. I'm not sure what the barrel does?

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That is a raised catch basin that is designed to keep the retention pond from over flowing as surface runoff (either off site or to an area where it could cause harm). The idea is for the swale to hold the water until the soil can absorb it, but in the event that there is more water than it can hold, a subsurface drainage system will kick it as the water pours into the catch basin. Sometimes these are also put in to keep the swale from getting too deep and requiring it to be fenced off (a fence is required for retention swales capable of getting 2' deep or more here in Massachusetts). Many comunities are now requiring this type of drainage solution rather than putting runoff directly into subsurface systems. The idea is that the plants will use excessive nutrients from the runoff rather than having it get into the groundwater which ultimately finds its way to ponds, lakes, streams, or bays causing algae blooms and other problems.

We commonly design these on commercial projects.

I would check with the local government authority on just what you can do with your drainage ditches. They are they to drain water, not look pretty. In our rural townships we have newbies to the country that fill the ditch in so they have flat surface to mow. This is illegal, causes drainage problems for their neighbors and can get expensive when they have to be restored. If you really don't want them, as the government agency if you can install pipe. At your expense, of course.