Streams add to the sights and sounds of a pond and serve as biological filters for a pond. While more complex to design than a pond they are well worth the time.
Before building a stream take a walk in nature and see how they run. Do they meander and wander? Do they gurgle and babble or are they tranquil and smooth? Does the flow bounce from one side of the stream to the other? How does this occur in the stream? Paying attention to the mechanics in nature will help you to replicate the look in your stream.
Whether your stream bubbles and churns or not is dictated by the elevation change. You need to have at least a 1 inch rise (ideally 3 inches or more) for every foot of length if you want more than canal.You do not have to live on a hill to make a babbling brook. A flat yard can have a bubbling brook as well. Remember a 12 foot stream only needs a 1 foot rise to to make noise. Of course the more rise the more you can do to add character to the water flow.
The stream channel is much like building a pond except it is only inches deep and longer. It will require a liner (preferably EPDM) that totally contains the water.
The depth of a stream is commonly four inches or more. When it gets less than that it has a tendency to grow filamentous algae more easily.
The width of the water channel in the stream should be no narrower than 8 inches after the rocks are added. Do not forget to add width and depth for the rocks you will be using for the sides and bottom. Add an extra 4-6 inches of liner on each side of the stream for adjusting the wall height and to compensate when the soil settles around stream. fold it down, creating a double wall. You can then pull it up as needed.
The amount of water the pump should produce should be from 200 to 400 gallons/hour for every inch of width. So an 8 inch stream should have between 1,600 to 3,200 gallons/hour running through it to look its best.
Nothing adds more character to a stream than bends and turns as they look very natural (especially when compared to streams that run in a straight line). Once you have laid the rocks that will form the sides fill in the cracks and holes with expanding foam (also known as Great Stuff in a red and yellow can). You can get it at any hardware store. This will keep the water going in front of the rocks and require less flow to achieve your desired effect. Allow it to dry for at least 4 hours and break away the excess. The foam will expand to more than 3 times its original size so allow for that or you will waste a lot and have more work to do to remove the excess.
Once the rocks are sealed in place it is time to turn on the stream and start placing the large rocks on the bottom. Start at the end of the stream and work your way back or you will be adjusting rocks twice. Make the water run at 90° to the length of the stream to create a more brook-like appearance. Allow the stream to widen for a short length, which will slow water down, then narrow it and the water will speed up in the narrow sections. Once you like the general appearance of the stream turn the water flow off, let the stream bed dry out and add expanding foam to the holes under the larger rocks. This will force more water to go over rather than under them. The exposed expanding foam will be covered with algae and silt in about 4 weeks so don't work too hard at hiding it.
Now comes the easy part; turning the water on and making the final adjustments. Then sit back and enjoy your work.