Average velocity of a river is simply how fast the water is moving, usually measured in feet per second (ft/s). The velocity impacts the types of organisms that are found in the water. Some aquatic organisms need fast-flowing, highly-aerated areas, while others need quiet pools to survive. Velocity also has an affect on the water column. Fast-flowing streams will hold suspended sediments in the water column longer, while quiet, slow-moving rivers will allow suspended sediments to settle out to the bottom quickly.

1. Use a tape measure along the stream bank to mark a length 10 feet long.
2. Position someone upstream and someone downstream.
3. Release an orange into the main current at the beginning of the marked length. (An orange works well because it floats more or less in the zone of maximum velocity just below the surface.)
4. Have someone time (in seconds) the passage of the orange from the beginning to the end of the marked length. The downstream person should yell when the orange floats by the end point.
5. Repeat this test twice and average the results.

In general, if the result is less than 1 foot/second the stream can be considered slow-moving.

Stream velocity, which increases as the volume of the water in the stream increases, determines the kinds of organisms that can live in the stream (some need fast-flowing areas; others need quiet pools). It also affects the amount of silt and sediment carried by the stream. Sediment introduced to quiet, slow-flowing streams will settle quickly to the stream bottom. Fast moving streams will keep sediment suspended longer in the water column. Lastly, fast-moving streams generally have higher levels of dissolved oxygen than slow streams because they are better aerated.