questions about making a dry river bed

When our tree guy (also lawn guy, just happens to be the company's tree expert as well) came over my husband was pointing out a spot in our yard that we really have trouble growing grass. It was a mud pit when we bought the house. After seeding it twice, we've finally got some grass growing. The lawn guy pointed out that while it's growing now, it will probably be a constant battle to keep it looking good since that spot gets basically no sun. He suggested not bothering to fight with that spot and rather turn it in to a dry river bed (duh, why didn't I think of that?). I really like the idea and I think I can get my husband on board as well. It's not a big spot (maybe 4 feet by 8-10 feet). I'd like to make a dry river bed with maybe a few stepping stones in it (bonus is that it could be where the dog pees rather than on our grass). So now for my questions:

1. How deep would we need to dig down and then fill up with river rock?
2. I feel like we'd need to put something down under the rock to keep the rock from eventually disappearing into the dirt and to keep the weeds from coming up through the rock. Would landscape fabric be the best bet?
3. How would we keep the rock from spreading out of it's spot (some type of edging)?
4. Anything else you can suggest to do or avoid doing?

This project will probably be one for next spring and I'm really hoping to find some river rock cheap/free on something like craigslist (we're not above doing the manual labor ourselves and my father-in-law has a trailer to haul it in).

To make this look right, I want you to picture a small stream, creek or something. A real one, made by nature.

Q. Is it flat with the surrounding soil? Or is it sunken in?
A. If it was flat, the water would not stay in the stream bed, so of course it is below the surrounding soil!
How to: Dig it out, and you can use the soil you generate to make something of a hill that the water comes from, sort of like a natural spring coming out of the ground. Rake the soil around so the hill looks like it belongs there, not just sticking up in the middle of nowhere. The deeper you dig it out the better it looks. If you only dug out 6", then filled that with rock, it would not be sunken in. Make a deep bed for the stream.

Q. Is it just a straight line going from point A to point B?
A. well, the boring ones are. The more interesting ones meander back and forth, perhaps split out around an island.
How to: Lay out a gently curving line with a garden hose. Then make some marks where the stream could be wider, and where you want it narrower. In narrower areas you might use a couple of larger boulders to confine the water flow, the same as you would see in nature. You could add a stone between these boulders to make it look like a water fall. (Of course dig out the soil so it really would flow like that) Where there are fewer boulders, the stream can spread out, flow more gently.
While we are on the subject, where does it flow to? Gotta go somewhere! Maybe enlarge the end into a pond-like area. Sort of like a river ending in a lake.

Q. What kind of materials are they made of?
A. Rock: The movement of the water has washed away a lot of the soil, and there are rocks. Not sharp, angular rocks, but softly rounded rock, washed by eons of water flow, and bumping down the stream, crashing into other rocks, getting worn away.
Q. Are all the rocks the same size, lined up, and orderly? Are the banks outlined?
A. The rocks are many sizes, some well sunken into the banks, and others have been tumbling around, and have landed here for a while, then they are moving on. There is not a sharp outline as such. Not a perfect, neat line.
How to: Install the largest rocks first (24" minimum diameter). These will be the defining boulders, where the stream changes direction, or a water fall happens.
Then install the mid-sized rock (12" avg. diameter). Some of these will look natural in the bank, and some in the stream, partially buried. In a smaller stream, this could be sticking up above the water line, sort of like an island. In a larger stream this would be the beginning of an island- the rock protects the island from getting washed away.
Then the smaller rock (4- 6" diameter). Do not scatter these equally all over. Make some areas fairly concentrated, and other areas more sparse.
The last will be the smallest rock, 2" or less. Just like the 4-6", some areas will have greater concentrations of this smaller rock (washed here by the currents) and other areas will be pretty sparse.

Q. What is also found around water?
A. Plants. Get some input about the right plants for the setting, but if it is too shady for a lawn, perhaps some ferns, azaleas, hostas and other shade plants would do well. Tuck these in tight to the rocks on the banks of the stream.

You are right to be concerned about weeds growing, and rock mixing with the soil. If you can find a good, soft landscape fabric, or filter fabric (ask at contractor's supply places, irrigation stores, places that specialize in draingae)
Dig the area out deep enough so you can put this in under the rock, including being able to dig in the large boulders. This is why you want a flexible material.

I agree whith all the info and hint, idea's etc from Diana, she really knows her gardens planning, layout's and materials to help suit the type of project needed.

I would recommend you try go to library to search for books on water gardening as these books dont all cover just making ponds, also IF in your area there are any natural streams, take pictures of this area, try look from one bank to the other as the stones or rock foundations are never the same both sides of a stream,
IF you have a bend in the river bank, then remember there are usually a larger gathering of stones there as when a stream is running in full strenght, it really does shift lots of stones of different sizes.

I've seen some very silly attempts at dry river beds and believe me, unless you are able to buy the right type of round or semi flat stones, various sizes and plenty of them, large stones for the purposed Diana pointed to and a good quality fabric or underlay then all your hard work will bring tears by bedtime as my grandmother would say.

I would search books from book store on the subject so you are armed with info, prep work, tools and how to start the project right through to the finnished work, you dont have to make this project of your as a copy of any pictures as by reading up on the subject you will be able to alter to suit your own needs.
I love those types of garden areas when done well, plant selection natural looking, but also remember that without good sun or part shade you will get some mosses etc taking hold, thes growth add to the attraction and naturalness.
I wish you every good luck and hope you achieve your goal, just be prepared to do, undo and redo till you get it right LOL.

Keep us informed of the progress as you go along.
Kindest Regards.
WeeNel.