Thursday, March 8, 2012

Flood Damage - Channel

The main creek channel appears untouched by the raging flood waters.  Clear water splashing over clean rocks is normal for this section of the creek.  The difference is that those may not be the same rocks that were there prior to the flood.  As the velocity of flood water increases, its ability to move loose rocks also increases.  At the peak of the flood, rock of various sizes was moving downstream along the entire length of the channel.  When the water slowed, the rocks fell back to the bottom. 
Some areas that had pre-flood bottoms of sand and gravel, now display clean bedrock.
Rock and gravel bars developed where water was temporarily slowed.  This is material that once formed the stream bed somewhere upstream.

The water has to slow even more before the sand settles out.  The Wild Turkeys must have run up and down the creek channel as soon as the water receded.   Maybe they represent the new scourge, Disaster Tourists who turn someone else’s misfortune into an opportunity for a family outing.

This is why it’s a poor survival strategy for Streamside Salamanders to lay their eggs beneath the smaller flat rocks.  It’s hard telling how far this rock traveled before coming to rest on its edge.  If eggs had been attached, they never would have survived the journey.

The most noticeable impact to the stream channel is blockage by debris. 

The blockage usually begins with the accumulation of a couple of large trees or logs.  These in turn catch smaller material until an almost solid structure is in place.  Water is forced to go around or go over.  Water coming over the top of a log jam will create a pool where the water falls on the downstream side.  Pools created in this fashion often contain the last open water before the stream goes dry in the summer.

Leaves and other bits of fine organic matter will quickly decompose.  Unless another flood replenishes the supply of loose material, the structure will be reduced to just the big logs.

In some situations, water is forced beneath the logs.  The pool forms upstream and below the obstruction.  The abundant cover associated with these pools attracts a variety of wildlife species.

Sometimes it’s hard to determine just how the pile-up was created.  The key is usually a tree that has fallen across the creek.  The fallen tree intercepts material being carried down the channel.  It’s difficult to imagine how an entire tree can find its way down such a narrow, winding channel.  In this log jam, the large tree on the right and the long Y-shaped tree with a root ball both came from somewhere upstream.  I hope we’re not destined to experience an even larger flood capable of moving this mess on downstream.

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