How to Dam a Creek. A dam is a structure that stops water from flowing. Sand bags and rocks are an easy way to dam a backyard creek. Alternate layers starting on the creek bed spot you want to dam. Stack as high as necessary. Here's another way to dam a creek using lumber.
Measure the width of the spot in the creek where you want to dam. Use the measurement to determine how many 2 by 6 inch or 2 by 10 inch wide pieces of pressure treated lumber you'll need to cross the creek. For a five foot wide creek bed, you must have a 10 foot long piece of lumber cut in half to make two 5 foot long boards. Get three extra 2 by 4 inch wide by 8 foot long boards to use for upright posts and bracing. Cut one of these in half. Now you have two pieces to serve as posts and two pieces to serve as braces.
Use a sledge hammer to drive the 4 foot long 2 by 4 inch boards down into the creek bed at each side, a foot deep. They should be secure enough to stand upright. Drive another board behind each one standing to act as a brace. The braces should be placed in the opposite direction of the flow of the water.
Figure the height you want to dam the creek. For a 3 foot high dam, if you use 2 by 6 inch wide pieces of lumber, divide 6 into 36 inches. That will give you the number of pieces you need for the height. Use the same steps if you are using 2 by 10 inch boards (divide 10 into 36).
Stack sand bags and rocks in an alternating fashion on either side of the wall you've built. This acts as an extra brace for the dam wall and helps to preserve the lumber so it will last longer when you dam the creek.