Monday, October 17, 2011

Autumn on the Middlebury River - Leaves in the Water

Autumn has come to Addison County.  Many of the trees along the Middlebury River are covered in colorful leaves.

Some of the leaves find their way into the river, where they float downstream, occasionally finding themselves clogged in 'leaf jams'.  I found myself wondering if this has an effect on water flow.

I've definitely seen large accumulations of leaves blocking some of the slower side channels of the river, as well as smaller brooks in nearby areas.  In the case seen to the right, the leaves had accumulated at a fork in the side channel, and seemed to be diverting flow from one channel to the other.

The little leaf dams do seem to slow down the water a bit, but when the water rises during storms, they are washed away.  In smaller streams the accumulated leaves probably do have an effect during high water, but its hard to say whether the little dams do much to slow floods.  The leaves probably do a better job reducing runoff when they are still attached to the tree.

In urban settings, autumn leaves have a much more detrimental effect - they tend to get caught in drains and cause localized street flooding.

There is another factor that can increase water flow when trees lose their leaves.

When leaves change color and fall off in the fall, the tree becomes dormant for the winter.  Each tree uses quite a bit of water every day when it is leafed out, and it can make a big difference when all the trees in a watershed 'shut down' for the winter.  In fact, in parts of California with very dry summers, some creeks can start flowing in November when the trees along them become dormant, even if no winter rains have fallen yet.  With more 'base flow' in the creeks and rivers, flood crests can be a bit higher.  This is one way Vermont was lucky with Irene's flooding - if the storm had come in November, like the 1927 hurricane, damage could have been even worse.

Soon all the leaves will have fallen from the deciduous trees along the river, and I'll get to watch ice start to form.  Meanwhile, you can click here to see more photos of this year's fall color in Vermont.

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