Monday, March 1, 2010

Tree in Creek

The only way to keep a stream from changing is to keep it in a picture. A stream is a dynamic entity that constantly changes its appearance. Ten years ago, this section of creek flowed over an 18 inch layer of gravel and mud before falling into the pool shown at the bottom of the photo. A tree falling across the channel began a series of events that are still changing the way this creek behaves.

The tree was dead when it fell and the trunk was straight. Initially, it rested about eight inches above the stream bed. Normal water flow was able to pass beneath this log bridge, but rain runoff was blocked and forced to come over the top. The force of the water coming over the top of the log dislodged the mud and gravel, creating a large pool. As the gravel bounced along the stream bed, more mud and gravel were kicked up into the moving water and carried away. The process slowed when larger rock pieces were encountered. The result was a deeper stream channel and the creation of vertical banks.

The pool still exists below the log, but the water lacks the energy required to move the larger stones, so the pool remains shallow. The newly exposed banks are not stable and are allowing the water to come around one side of the log. Through the years, the water soaked log has bowed and followed the descent of the stream bed. This phenomenon greatly increased the log’s effectiveness as a stream changer. It won’t be long before the log decomposes enough to break apart and be carried downstream.

When the tree first fell, heavy water flows would accelerate the speed of the water moving beneath the log. The fast moving water created an upstream extension of the pool and created a slight drop resembling a mini waterfall. Water passing over this drop increased slightly in speed and carried away some of the mud and gravel. As this process continued, the elevation drop moved upstream. Eventually, the mud and gravel was completely removed from the stream.

A single fallen tree caused several hundred feet of stream in both directions to change from a mud gravel bottom to a clean rock bottom. The rock bottom is more typical of small streams in the upper portion of this watershed and there seems to be a greater abundance of organisms now living in this stretch of stream. There are more trees that look as if they might fall in the future. It’s possible that another tree could fall and start the process of reestablishing a mud and gravel bed.

1 comment:

  1. I LOVE this: "The only way to keep a stream from changing is to keep it in a picture."

    Great post! It could be a metaphor for life and how we encounter obstacles that cause us to change in order to keep on going :)