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 many
fold. 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 river banks. 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 flood-plains. 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 bath tub rings, with algae only
growing on the upper half.
Poorly planned development disrupts the cycle of streambed renewal.
Where stream banks are bare, erosion can be a big problem. Soil lost
from over-grazed or over-cropped 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. With respect to Joni Mitchell, there’s
a lot of room for ingenuity between paradise and a paved parking lot.
Declan McCabe teaches biology at
Saint Michael’s College. His work with student researchers on insect
communities in the Champlain Basin is funded by Vermont EPSCoR’s Grant
EPS-1101317 from the National Science Foundation.
This article was written for Northern Woodlands Magazine's Outside Story and first published on August 3 2015
The stonefly was photographed at Saint Michael's College by Erin Hayes-Pontius
and is available from Wikimedia Commons under a CC BY-SA 3.0 Creative Commons license.
© by the author; this article may not be copied or reproduced without the author's consent.
