Low Flow Database

Structural Alterations

Streamflow in unaltered systems varies naturally during the course of the year. In fact, these pulses from high water to low water and back again, known as the natural flow regime, actually create the dynamic and interdependent habitats in and around flowing waters and fosters the biological life that flourishes in them. This natural flow regime refers to the magnitude, frequency, duration, rate of change and timing of streamflow that would have occurred without human modification of a river or its watershed characteristics.

In Massachusetts, spring is the time of high water, when melting winter snows and spring rains flood watersheds, streams and, ultimately, rivers. Low flows occur naturally in late summer and fall when plants are taking in a lot of water to support their full load of leaves. Native aquatic plants and animals have evolved to take advantage of a particular river’s natural flow regime and frequently suffer when it is altered.

Effects on Habitat

As streamflow decreases, many physical changes occur that can negatively impact aquatic habitats. First, water levels drop below the banks where overhanging roots and woody debris, during normal flows (below, left), offer excellent habitat for many aquatic species, including fish, inver-tebrates, reptiles and amphibians. The banks are then exposed to desiccation, wind and freeze/thaw cycles (below, right), which can cause increased streambank erosion and sedimentation downstream. These processes reduce the amount of stable bank habitat that is normally used by many invertebrates and semi-aquatic mammals—such as beavers, snakes and otters—and smother gravel and cobble streambeds that are critical nesting habitats for fish.

As water levels recede further, riffles may dry out (see picture below), cutting off migration and movement routes for aquatic species as well as cutting off the flow of the river that some species require to survive or reproduce successfully.

Riffles also serve as some of the most productive areas for aquatic macroinvertebrates, such as mayflies and stoneflies, that are a major source of food for many fish. In extreme cases, rivers may dry into a series of isolated pools (see picture below). Aquatic organisms are then concentrated into small spaces with little cover and increased predation risk.

Effects on Water Quality

In addition to shrinking habitat and availability of cover, declining streamflow is associated with several water quality concerns. As water levels drop, streams and rivers become shallower, with less area shaded by riparian vegetation, and the waters generally experience a rise in temperature. This problem can exacerbate the naturally occurring summer conditions in Massachusetts—low flows and high temperatures. Declining groundwater levels can also cause cool springs and groundwater seeps to cease flowing, which further increases the temperature of many streams. Many species of fish require cool water temperatures even in summer, and they can be seriously stressed by high temperatures and loss of cool spring refugia.

Dissolved oxygen concentration is partially dependent on water temperatures and the amount of turbulence in rivers. Thus when temperatures increase and rivers slow or cease flowing completely, dissolved oxygen can fall to critical levels for many aquatic species. Low dissolved oxygen concentration is often blamed for extensive fish kills during extreme low water periods.

In streams and rivers with large loads of pollutants, low water levels can cause these pollutants to be concentrated in smaller volumes and cause health concerns for aquatic species and humans alike. Low water levels also concentrate nutrients such as nitrogen and phosphorous which can lead to unpleasant algal blooms in many water bodies (see picture below).

In Massachusetts, most low flow concerns center on summer conditions because that is the time when streamflow is naturally lowest and human water use is highest. But low winter flows can also cause serious problems for many aquatic organisms. River banks and mud flats are the winter hiding places for many aquatic and semi-aquatic species that depend on a covering of water to protect them from freezing conditions. When these habitats are exposed to freezing temperatures and dry conditions, many species, including mussels, frogs and turtles, may experience increased overwinter mortality. In some smaller streams, low water levels may allow ice to form all the way down to the stream bed (see picture below), causing aquatic organisms to either move further downstream or be frozen solid.