Friday, May 20, 2011

How life copes with floods in streams and floodplains

How life copes with floods in streams and floodplains

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[Please note: this piece ran originally on June, 12, 2008, when flooding was more severe in the Midwest, but we pulled it out as an archive spot for the radio in light of this year's flooding in the South.]

This spring’s heavy rains in the Midwest have resulted in all manner of difficulties for people, from the tragedy of lost lives to the pain and hardship of flooded homes and farm fields. This is where humans need to come together and help one another out.

The other forms of life that inhabit streams and stream corridors are also coping with the high waters, some more easily than others.

At the bottom of freshwater streams, unusually strong flows may deposit large amounts of sediment on top of mussels, which live hunkered down in sand and gravel. Mussels cannot survive if they stay buried completely, so they must make their way up to the new surface of the streambed in order to survive. Where the streambed is scoured away in flooding, some light-shelled mussel species may be swept up in the current, and then left high and dry when floodwaters recede. After the Mississippi River flood of 1993, scientists from the Illinois Natural History Survey found stranded mussels at densities of up to 1 per meter in farm fields more than a mile from the river. Individual mussels stranded on dry land can’t survive, but the problem of stranding doesn’t seem to have a long-term impact on the overall health of mussel populations

Creatures that live on and near the streambed--including crayfish, insect larvae, and other invertebrates--need access to stable refuges in order to ride out flood pulses. Such refuges may include rocks or logs that remain in place, as well as undercut banks and other streambed irregularities that create pockets of reduced current.

Farther up in the water column, fish use their mobility to cope with rising waters, and they even benefit from floods in some circumstances. Fish may ride out a short-term flood by seeking pockets of water that are protected from strong currents, or moving up into smaller tributary streams. In more extensive, long term floods fish take advantage of the opportunity to move out into areas that are not normally submerged. Slow moving water on a floodplain quickly becomes rich in microscopic life, which attracts minnows and other small fish. And where small fish go, the larger fish that feed on them follow. The rich soup of flood waters also offers a variety of seeds from trees and other plants, as well as drowned insects, and more. If water persists in the floodplain long enough, some fish will even take advantage of the opportunity to spawn there.

The trees common to floodplains are also adapted to occasional high water in fascinating ways. Willows, for example, are extremely flexible, so that they bend in strong currents rather than breaking. A willow that’s bent over far enough to be buried with sediment can even send up branches that then emerge from the ground like new trees. So when you see a straight line of little willows on a gravel bar, you may actually be looking at shoots coming up from the trunk of a tree laid down by a flood.

While flooding in the Midwest was once an entirely natural occurrence, it isn’t anymore. Our streams now rise higher and faster than they used to because we are so good at moving rainwater off of the land quickly, with drainage for agriculture and paving and building in cities and suburbs. The real challenge to us now is developing landscapes that will once again hold some water back.