Friday, January 29, 2010

Creek Swirl

You don’t often see streams, under natural conditions, make a 90 degree turn. This section of creek comes in from the right, makes a sharp angled turn and then flows away. In the process, water builds up in the turn and forms a concentration of swirling water. A well proportioned circular pool results.

Recent events have worked to intensify the forces at work in the pool. A fallen tree, combined with debris carried from an upstream neighbor during a flood event, has formed a low dam across the channel. During runoff events, water built up behind the dam accelerates as it comes over the top. This extra energy and turbulence allows the water to scour loose material from the stream bed. The result is a couple hundred feet of clean stone bottom.

The dam catches floating debris and halts the progress of large particles moving along the stream bed. This accumulation strengthens the dam, but forms a long section of stream with a sand or mud bottom. This is a common occurrence that increases the diversity of stream habitats.

A new tree has fallen across the dam. When it falls to the weight of decomposition, it will add its mass to the structure of the dam.

The newly fallen tree didn’t even have the courtesy to break. It looks as though the trunk suddenly became limber and let the tree recline without cracking.

This is why the creek has no choice but to turn. A limestone base overlain by thick shale will stop any flow of water. Besides the barrier of rock, there is quite a water flow that follows the twin gullies and cascades over the exposed shale. This flow helps to accelerate the water circulation in the pool.

This area has been remarkably stable over the past 20 years. The curved tree trunks are indicators of earlier soil slippage. When the trees were young, sliding soil caused them to lean down hill. The soil then stabilized and the trees were able to redirect their growth upward. The hillside is still very fragile and a change to the stream channel below the shale outcropping could cause the soil to slip once again. Understanding the historical events that brought a site to its current condition is essential to predicting the future results of your current management activities.

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