Low Flow Database

Dams

Dams and Their Effects...

On Streamflow
For thousands of years humans have built dams to control the flow of water. In Massachusetts, Native Americans built small dams, or weirs, to corral fish long before European settlers arrived. The colonists wasted no time in erecting dams to power grain and paper mills, and in the 19th century, textiles mills were constructed across the Commonwealth as part of the Industrial Revolution. Today, Massachusetts has an estimated 3000 dams over 6 feet tall and numerous, though uncounted, smaller structures blocking streamflow on its rivers and streams. This legacy of streamflow manipulation has left its mark on our aquatic resources by changing the natural flow regime in many rivers. Depending on the details of a particular dam and its management, changes in the natural flow regime can lead to changes in water quality, habitat condition, and aquatic plant and animal life.

On Water Quality
As water is held behind a dam the surface layer is warmed by the sun and sediments and organic particles that were suspended in the water column settle out on the stream bottom. In the bottom layers, this organic muck decomposes, consuming oxygen in the process, and can lead to anaerobic conditions in the deeper areas behind many dams. In addition to oxygen problems, the sediments that settle out can literally fill up the entire area behind a dam in a matter of mere decades, in some cases.

Water that is released over the dam from the surface layers will often raise the temperature of the stream below the dam, which can cause added stress to downstream aquatic life that may depend on cool water temperatures. Water released from the lower layers can actually cause fish (and other organism) kills if it is severely oxygen depleted.

On Habitat
The most obvious change in aquatic habitat quality caused by dams is the conversion of flowing water to standing water. Many native river organisms are in some way specialized to depend on flowing water conditions and cannot survive in standing water conditions (see Aquatic Life below). In addition, the physical structure of the dam itself prevents most aquatic organisms from moving freely up and down the river. Even when fish ladders are installed, many organisms are not able to navigate the passage and are trapped in a single stretch of river. For organisms with large home ranges or migratory needs for feeding or reproduction, barriers to movement are substantial obstacles to successful reproduction.

Changes in the amount of water and suspended material cause changes in the physical shape of aquatic habitat both near dams and farther downstream. For example, water that is released out of a dam typically scours new areas and causes unstable banks just downstream. As sediment is eroded from the unstable banks or scoured from the streambed itself, it is carried far downstream, where it is deposited in new areas and may smother eggs or benthic invertebrates.

Dams can also cause changes in the rate of water level fluctuation in a river. Some hydropower dams operate in “hydropeaking” mode to provide energy when it is most needed. Water is held behind the dam during hours when energy demand is low and released only when energy demand is high. While this may sound like an intelligent way to provide energy on demand, it means that the river downstream of the dam is exposed to near-dry conditions while water is filling up behind a dam, followed by large surges of water released to produce energy. These fluctuations can occur several times a day below some hydropower dams! The graph below shows the effects of a hydropeaking dam on the Mumford River at East Douglas . Notice how streamflow declined as water was stored behind the dam (falling limb of each peak) and streamflow increased as water was released over the dam to generate power (rising limb of each peak), particularly from July through November.

On Aquatic Life
All of these changes to streamflow, water quality and habitat cause added stress to plants and animals that are adapted to the natural flow regime. In some cases, organisms can simply shift to a new, more suitable habitat, such as the migration of riparian plant species further down the streambank when chronic low flows occur. Some organisms, such as many benthic invertebrates, simply cannot maintain their position in the stream in rapidly fluctuating water levels and are carried downstream. In other cases, adult organisms can survive the changes in their environment, but they cannot reproduce successfully. For example, trout require clean, well-oxygenated, shallow gravel areas to successfully lay and hatch their eggs. In rapidly fluctuating water levels, low dissolved oxygen conditions, or turbid water full of sediment from eroding banks, these eggs will never hatch. Still other situations can cause changes in growth patterns, particularly in juveniles.

Finally, all of these responses to changes in the natural flow regime can eventually lead to changes in the structure of the aquatic community. River specialist species are replaced and out-competed by habitat generalist species, which can thrive in a wider range of environments. Dams also cause populations of species to be isolated from one another. This change in community structure can lead to local extirpations of some river-dependent species because the isolated areas can no longer be repopulated from individuals travelling up or downstream.

Habitat specialists, like the Baetis mayfly (left) depend on flowing water to survive. Habitat dependents, like the white sucker (center) can survive in ponded conditions, but cannot successfully reproduce without flowing water. Macrohabitat generalists, like the green frog (right), can survive and reproduce in both flowing and ponded habitats.

Permitting & Regulations

Hydropower dams are licensed by the Federal Energy Regulatory Commission (FERC). For projects with a generating capacity of 5 megawatts or less, FERC will generally issue a waiver with conditions based on recommendations from state and federal agencies. Larger projects go through a permitting process yielding 30- to 40-year permits. Either of these processes can result in water management conditions to benefit, or at least reduce the negative impact to aquatic life.

For dams that do not generate power it is more difficult to impose streamflow requirements. Projects that involve withdrawing water from the stream may need to file a Water Management and/or Interbasin Transfer permit, either of which could theoretically be used to impose flow management conditions. However, this has rarely occurred in Massachusetts, and most small dams do not have legal flow requirements.