This is how I do it. Other opinions will vary.
Paint the bottom. Glue in the larger details like fallen trees and rocks. Add sand of the color of your choice. I use a spoon and tap the handle to apply it lightly. Brush any sand off the rocks and trees. If the sand is uneven, you can tap the layout around the stream bed to get the sand to settle down and fill in around the base of the larger rocks. Apply some alcohol from the edge or more hidden part of your stream bed. Use an eye dropper. Then add the white glue water mix the same way. You need enough to glue it all down so it should be saturated but not floating. Let it all dry for a couple of days.
Needless to say that the bottom of the stream bed should be sealed.
If there are any bare spots, come back with full strength white glue and paint those areas, then sprinkle on some sand on them and let dry. Then vacuum up the loose stuff.
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
QRNational4101how can i make the dry creek look as if it has recenlty had a lot of water flowing through it, an everything washed down stream look?
I modeled a wash to appear as if the last rain wasn't too long ago.
I concentrated finer material toward areas where water would deposit it, and airbrushed some Floquil Railroad Tie Brown to darken the color. The airbrushed "mud" also accumulated on the larger rocks and debris in the path of the water, plus the walls of the culvert.
Here's a photo I took in the Grand Wash at Capitol Reef National Park. Several days before I hiked through here, there was a flash flood perhaps six feet deep. There was still-wet mud in only sporadic locations, but plenty of evidence of it dried on the shrubs and rocks. You can see a brownish cast to the leaves on many of the plants above. This sort of thing can also be represented by airbrushing or other means.
As with any scenery modeling, use photos as much as possible and represent what you see in them for best results.
Rob Spangler