Take a look at any Connecticut River tributary in August, and you’ll be hard-pressed to remember back to April when melting snow and spring rains filled it to its banks. At Spring levels, it would have been a struggle to wade across; with each tentative step, you’d be fighting the heavy current. Today, you can surely find a spot where you can pick your way from rock to rock and cross to the other side without even getting your feet wet.
That seasonal change, so dramatic for visitors to the river, can be life threatening to the animals that live there: trout, smaller fish like dace and sculpins, and the macroinvertebrates they feed on.
Low water level means high water temperature. All trout require cold water, and of the three species in the Northeast, the brook trout (the only native) is most susceptible to warm water. More than a day or two at water temperatures above 77 degrees F and brookies will go belly up. Rainbow trout and brown trout were imported to the region a century ago because they can handle slightly warmer temperatures, but the preferred temperature for all of them is within a few degrees of 60 degrees F.
By August, temperatures in the slower, low-gradient tributaries are generally in the danger zone, which forces trout to congregate in stretches of non-lethal water. This was demonstrated to me one hot summer day wading the Waits River with three Vermont fisheries biologists conducting their annual census of trout populations.
In one 600-foot stretch of the river, despite it having the classic combination of riffles and pools that all fishermen associate with good trout habitat, there were only two spots that held trout. The first was where a tiny brook 12 inches wide met the river and pumped a steady stream of cold water into the head of a bend pool. Lined up tight to the bank like children at a drinking fountain was a pod of wild brookies, ten of them in all – not surprising since the water temperature elsewhere in this stretch was 73 degrees.
In the next pool upstream, there were a similar number of brookies along with a small brown and some hatchery rainbows camped out in a three-foot-deep pool in the shade and shelter of a blown-down spruce. If it weren’t for the cold brook and the mature spruce that had fallen into the river, there might not have been a single trout in this stretch of the Waits.
Living in pods like that is stressful for trout, which are naturally territorial. When the water cools at night, they reclaim their normal territory to feed.
Low water also forces the macroinvertebrates to move. As long as the drop in water level is gradual, many of them have the opportunity to crawl or swim to the safety of an underwater rock in the channel. While some – especially the burrowing insects – will be left high and dry to die, the rest will be concentrated in the section of river that is still wet. Stoneflies are particularly adept at moving when it becomes necessary. Even freshwater mussels – though at a snail’s pace – are mobile.
Some rivers are worse than others in funneling all their life into a narrow summer channel. Those that run for miles alongside roads tend to ebb and flow dramatically. Rain falling on impervious road surfaces goes directly into the river, causing a surge in its level without adding a drop to the groundwater. Consequently, in periods when there is no rain, little groundwater is available to seep into the river.
Those rivers that are fed by springs and that flow through forestland are likely to maintain a more uniform flow and sustain much more life. Detritus (leaves, twigs and other organic matter) is the building block of life for phytoplankton and zooplankton, which feed the macroinvertebrate community. When a river can stay bank full, and there are eddies churning detritus, the river is like a rich stew. But when the river dries up to a narrow channel, the meat and vegetables are left to bake on the exposed cobbles and sandbars, and the river offers little more nourishment than stone soup.
That’s why fisheries biologists are universal in their recommendation to maintain and enhance forested buffer strips along all rivers, streams, and brooks. Trees are an essential part of the river system: they provide a thick mat of roots to keep soil in place; they provide a steady supply of organic matter; they help recharge the groundwater; and they help keep the water clean and cold.
Forest cover helps, no matter how small the stream. The feeder brook that provided a life-giving shot of cold water to the trout in the Waits River did so because it flowed through continuous forest cover.