Dams are constructed for a variety of reasons. Historically, many were built for water power to run mills. Later, many of these same mill dams were used for things like hydropower. Other dams were built for reasons such as navigation, recreation, lake level control, or aesthetics.
Some dams are very small while some very large (more than 100 feet tall). However large or small they may be, all dams have effects on our rivers and streams. These effects vary from dam to dam depending on its size and operation.
The first and foremost effect dams have is blocking upstream fish movement. Unless the dam has specific features built into it to pass fish, fish cannot move upstream. Dams negatively affect fish and their ability to reach critical upstream habitats necessary for spawning or different life stages or seasonal conditions. In some cases, this can be a positive – for instance when trying to block undesirable species such as sea lamprey or invasive carps- but dams can also block very desirable species such as salmon and trout.
“It’s important to note that the movement of fish is not the only thing that can be interrupted by dams,” said Kyle Kruger, a fisheries biologist with the DNR’s Habitat Management Unit. “Stream processes, including movement of water and sediment, can be affected by dams as well.”
Kruger explained that streams normally carry sediment and organic matter downstream – such as wood or vegetation – in balance with the energy of the stream. This weathering of the landscape is normal. However, dams create changes in the river which make conditions that don’t support the normal process.
As a primary example, dams create impoundments that slow the water down. As water slows, so does the energy in the stream that is carrying sediment and wood, thus causing sediment and wood to settle in the system. You can see this effect by the development of sediment deltas at the top end of an impoundment. Once water passes over the dam, the height differential between the impoundment and the river downstream creates additional energy. This creates a situation where the stream can be more aggressive at eroding the downstream stream bottom or banks until the balance between the energy of the stream and the materials it is carrying is restored.
Dams also change habitat types. By building a dam, a stream or river is converted to an environment more lake-like than river-like. This conversion may make the impoundment unsuitable for fish that formerly inhabited the river and make an environment that is suitable for fish that would not ordinarily live in the river. For example, many trout species are adapted to riverine environments and do not thrive in warmer lakes.
“An associated impact is that fish more suitable to the pond environment can be passed over the dam, thereby introducing them into an incompatible environment and resulting in weakened condition or death,” explained Kruger. An example of this is largemouth bass that may thrive in an impoundment but not in a river.
Dams change a river’s flow. Some dams operate as hydropower “peaking projects”. Peaking means they produce their power during the period of time when most power is used by consumers, or during “peak demand”. These projects, in essence, turn the water on and off at different times during the day. Historically, these types of projects could create a dry river for part of the day, and flood flows for part of the day. This dramatic change in flows on a daily basis is very disruptive to aquatic organisms. Today, there are very few peaking projects in Michigan and, for those that still exist, they either have minimum flow requirements that help protect the river, or their water flows immediately into a downstream impoundment which helps to maintain the water level in the river.
Another effect of dams on rivers is alteration of temperatures. The potential effects on temperature come in different ways. The first way is that by slowing down the water and forming impoundments, the water is exposed to the sun for a longer period and the resulting warmer water is discharged downstream of the dam. On some rivers this effect may be minimal, but on coldwater streams the water can be warmed up to a point that it is harmful to fish species such as trout and salmon.
“Another way dams effect temperature is by shifting the temperature regime,” Kruger said. “By this, if the impoundment is large, it often takes a long time to either warm up or cool down on a seasonal basis. This results in water temperatures downstream of the dam that are either warmer or colder than would be expected for any given time of year. There can be as much as a month delay in the temperature changes in a river system if the impoundment is very large. These temperature shifts can cause delays in fish spawning runs that eventually result in eggs hatching at the wrong time and unsuccessful natural reproduction.”
At dams that are hydropower projects producing electricity, there is also the effect of fish being pulled into the powerhouse by the water flow (entrainment) and passed through the turbines. Depending on the design of the powerhouse and generating equipment, fish passing through hydroelectric project turbines can experience anywhere from 5% to 100% mortality.
While dams can provide good things like electric power, recreational opportunities, and scenic landscapes, they are not without their drawbacks. For more information on how the DNR helps to manage dams in Michigan, visit Michigan.gov/dnrdams.