Thursday, March 17, 2011

Start of a New Creek Channel

Each year brings new changes to the creek. Windy weather has knocked a lot of dead branches from the trees and the flood water has carried them down the creek. This means the appearance of many temporary dams.

The dams force water to leave the creek channel and flow overland in search of another outlet. Dams that persist for a long enough period can cause the water to create an entirely new channel. This is a natural process that helps maintain a diversity of ecosystems in and near the creek.

The flood water doesn’t do much to benefit the plants that happen to be in its path. Flooding of lowland streams causes deposition of material that can improve the health of the soil, but that cannot be said for conditions found in the upper tributaries. Floods in these high gradient streams gather up all of the leaf litter and crumbly topsoil and carry it away. It’s not hard to determine which way the water was flowing.

Creation of a blockage such as this, follows a fairly typical pattern. It begins when a tree falls across the channel. The trunk of the tree forms a bridge beneath which the water flows. Eventually, a branch carried in the flood water is too large to fit beneath the bridge and it becomes stuck. The branch restricts the flow and acts as a strainer that catches more floating debris. This process of small material catching even smaller material continues until a nearly solid obstacle is formed. The water is forced to either going over or around the obstruction. Going around begins the development of a new stream channel.

Oils from decaying organic matter form froth on the water’s surface. As it reaches the blockade, the oil concentrates into bands that form a bow in the downstream direction. The bow illustrates the slightly more rapid current in the center of the channel. Water tries to adhere to anything it touches, so water flowing near the bottom or sides of a channel moves more slowly because it tries to hold onto the rocks and soil. Water in the center of the channel has nothing but more water to hold on to, so it moves along at a more rapid rate.

Redirection of channels is an old story in the creek. Not far above the obstruction is the site of an earlier channel relocation. The silty bank on the left is the location of a former stream channel that was blocked by a fallen tree. That channel is now nearly full of sediment and shows a variety of interesting little ecosystems along its length.


  1. Very interesting. It would seem science teachers would refer their 7th and 8th grade students to your blog. I remember covering these subjects about then in school, but I am learning a great deal more here.

  2. Hi Steve....interesting ..about the froth and how it is formed...I guess I never have thought about there being oils in the decaying process!!
    I am glad your back to keep me on my toes about whats going on around me!! : }}

  3. Lois - When my kids were in elementary and junior high school, I went in as a volunteer once a week to do science lessons. Stream formation was one of the things they studied in 6th grade. I took in a sheet of plywood and many buckets of sand and we spent part of a day running streams of water through sand layered on the plywood. By varying the amount of water and steepness of the board, we could demonstrate how creek channels formed. The teacher said that was the third messiest activity they had ever had in the classroom. Third place didn't make me feel bad since some of my earlier activities already held first and second places.

    grammie g - People often forget that plants can contain a lot of oil, even though they either eat or cook with vegetable oils every day. I don't want to be responsible for you having another fall, so I think you should get off of your toes and use your whole foot for stability.

  4. Excellent post. We have a small creek in our backyard with a muddy "floodplain". We had some good rains earlier in the week and the creek formed a new channel a few feet away- just after I planted a bunch of snowberries on the bank! Hopefully the new plants won't be washed away.

  5. Thanks for pointing out plants have oil. Duh! Whenever I've seen oil on surface waters, I assumed it was contamination of runoff from roads and such. No longer. Thanks for being informative as always. Still waiting to hear what happened to you over the winter. Glad to know you're alive and kicking.

  6. Hi, Mike. You can never trust a creek. Hope your plants survive.

    I used to react the same way, Katie. To paraphrase Tarzan, "Oil Bad".