Different Speeds in Stream Current

Observation by Todd Sjodin:

While doing my makeup assignment for my Biology Laboratory I was down by North Fork taking pictures by the stream and noticed the differing speeds in current throughout the small part of the stream in view. A part of the stream was almost completely stagnant and just a little way farther down stream it was flowing like crazy. I gave it some thought as to why this might be happening although I'm by no means sure of these assumptions. My first thought was that because it's the start of winter and parts of the stream are freezing it could disrupt the flow in certain areas and this was what I settled on for a while. The more I thought about it the less sure I was that this was actually causing it as freezing would just affect the very top of the water and shouldn't disrupt the flow too much. This thought led me to my current prediction which is that differing depths in the stream have differing effects on the current. For example, in the deeper parts the water becomes harder to move and pools which causes the current to slow in the deeper parts wherein the shallower parts all the water moves at once and is never in one place which causes a much faster current. A simple observation I know but it was on my mind after doing the makeup assignment so I thought might as well share it.

Crane Johnson with the River Forecast Center writes:

You are on the right track with your hypothesis that different depths have differing effects on the flow of water. Here is a link to a hydrology lab exercise which has a great figure that shows the changing water velocity (speed) with depth. Link: http://snobear.colorado.edu/Markw//IntroHydro/10/lab03/lab03.htm. In general, water towards the surface and near the middle of the stream will have a high velocity (faster current). I think the changes you saw upstream to downstream were due to riffles and pools along the stream. Pools form upstream and behind areas that block the flow of the stream. Pools can be formed by a rock ledge across the stream, trees or boulders blocking flow or a section of the stream that is flatter (lower slope) than other areas. Great observation about the ice along the edges. Ice does affect all of our streams and rivers in Alaska. It makes the surface of the stream rough (compared to open air) and slows down the water beneath the ice. Often we see a rise in the river as an ice cover forms due to the lower water speeds just below the ice cover.