The Outside Story

The Outside Story: Keeping it clean downstream


By Declan McCabe

In peaceful streams, aquatic macroinvertebrates such as crayfish, stoneflies, and caddisflies travel over and under submerged rocks, foraging for other invertebrates, leaves, and algae. When rain falls, their world turns upside down. At first only the surface is disturbed, but before long, runoff reaches the stream and increases its flow manyfold. Silt and sand blast every exposed rock surface. At peak flow, boulders are propelled downstream by powerful currents.

How do small creatures survive such crushing chaos? They hunker down. Water-filled nooks and crannies extend deep below streambeds and far beyond riverbanks. These deep interstices provide a safe haven even while turbulent water pulverizes the riverbed, comparable to a storm cellar in a tornado.

Storm water has great destructive potential, but it also flushes and cleans aquatic habitats. Riverbeds are rejuvenated when sediment is flushed from the bottom and deposited on floodplains. Algae and bacteria grow back rapidly on the scoured rocks. Macroinvertebrates climb out of their cellars and return to foraging. The cycle of storm, recovery and regrowth is the natural state.

You can see the effects of this cycle yourself by looking at river rocks. In a healthy stream, you’ll find rocks perched on other rocks, with the streambed visible under the water, and little silting. Although, just after a storm, the rocks may be scoured clean, they’ll soon develop a slimy covering of algae and host a diversity of tiny creatures.

In polluted streams, however, you’re likely to find something different. River rocks may be embedded in silt, and when you pick them up, you’ll find that they’re wearing bathtub rings, with algae only growing on the upper half.

Poorly planned development disrupts the cycle of streambed renewal. Where streambanks are bare, erosion can be a big problem. Soil lost from overgrazed or overcropped land ends up in the water, where it plugs the streambed nooks and crannies. Imagine a concrete truck unloading through your window, and filling your home with a solidifying mess. Only the hardiest of invertebrates survive these conditions, and the whole riverine food chain can be affected.

Traditional paving and buildings also create problems, as impervious surfaces dramatically increase the volume of water sent straight to streams. Formerly small, cool, perennial streams can become torrents of unnaturally warm water. Channels become deeper and eroded materials are deposited in stream beds. Since rain doesn’t reach the ground underneath the pavement, ground water can become depleted, and the streams may run dry between rain storms.

Farming or urbanization won’t disappear, but there are ways to intelligently develop landscapes to better protect streams. For example, St. Michael’s College, where I work, has installed a system of curbed parking lots connected to rain gardens. These are shallow, gravel-lined depressions, strategically planted with vegetation that tolerates occasional submersion. The rain gardens easily absorb water from typical rainstorms and can even contain all of the water dropped during 100-year storm events. St. Michael’s has also replaced many impervious sidewalks with attractive, pervious, bricked footpaths. Roof water from the gymnasium runs into deep gravel beds. Runoff from recently constructed roads collects in an underground tank. All of these systems drain gradually into the ground, drastically reducing the downstream potential for erosion. The recharged groundwater keeps a small perennial stream flowing to the Winooski River even during dry spells.

There is no doubt that people affect stream macroinvertebrates and the fish that they sustain. We do, however, have the choice to protect our streams by thoughtfully managing our impacts and reducing erosion in urban and agricultural settings. There’s a lot of room for ingenuity between paradise and a paved parking lot (with respect to Joni Mitchell).

Declan McCabe teaches in the biology department at Saint Michael’s College. He works with student research teams to better understand the insect communities in Lake Champlain and its tributaries. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation:

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