New stream environments created by flooding

wappinger creek
Flooding benefits stream life by transporting wood and fallen leaves downstream.
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Ice out at last! The East Branch of the Wappinger Creek, which runs through the Cary Institute's property, remained blanketed under a thick layer of ice and snow most of winter. Not until mid-March did the frozen cover begin to melt away faster than it reformed.

This melt water, supplemented by melting snow from surrounding hills, raised the creek level until it filled its banks and spilled out into side channels and lowland fields. Flooding of the East Branch can occur in response to storms at any time of the year, but is most common in the spring when snow melt is accelerated by warm temperatures and heavy rains.

The flow of water shapes a stream's physical and biological characteristics. Moving water constantly alters the streambed as silt, sand, gravel and even boulders are rolled about or swept away by the current. The physical movement of the streambed threatens stream organisms with being crushed or swept downstream.

Yet not all is lost in even the worst flood! Stream organisms have evolved adaptations for life in such harsh conditions. Some insects adapt to flooding by burrowing into the stream bed or clasping tightly to structures so as to not be washed away. Fish and other creatures can move out of strong currents to more protected areas. Cary Institute aquatic ecologist Dave Strayer has found that populations of freshwater mussels depended on protected areas to survive. Mussels, which can remain buried in the streambed for more than 30 years, live mainly in the small stable bits of streambed that are not disturbed even during floods.

Some organisms can quickly rebuild their populations after floods, with some even using disturbance events to their advantage. Spring conditions are perfect for the rapid growth of algae (a phenomenon called algal blooms). These microorganisms make their own food using sunlight. They take advantage of ample sunshine reaching the stream before trees leaf out to rapidly recolonize bare surfaces scoured and polished by tumbling currents. Algal growth can be further enhanced when the insects and other creatures that feed on them are washed away by floods. Thus, flooding can refresh the base of the food chain that, in turn, helps feed the stream ecosystem.

Flooding may also benefit stream life by transporting wood and fallen leaves downstream. These forest products provide important nutrients and structure in stream ecosystems, building streambed and floodplain habitat, and providing food for creatures that feed on organic matter. During floods, large branches and even whole trees can be moved long distances downstream. Strayer has been tracking the fate of trees felled by storms on the Cary Institute's property in 1987 and 1996. He found that the Wappinger Creek can move large logs more than a mile downstream during large floods.

Every flood event creates an opportunity to see a new stream environment formed when the old stream and its inhabitants are reshaped by the churning waters. Low water periods during summer are great times to explore the impacts of past flood events and see how materials were moved and redistributed. Remember the role that branches and logs play in sheltering stream organisms. Avoid disturbing these important habitats unless absolutely necessary, such as when roads or bridges are threatened. Permits may be necessary to alter the bed or banks, so contact your local NYS Department of Environmental Conservation office for details before working in and around streams.

Cary Institute of Ecosystem Studies | Millbrook, New York 12545 | Tel (845) 677-5343

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