Water Level Management
Related page: Hydropower and Dams (including petition to set water levels/minimum flows)
Some Common Questions
The Lakes Assessment Program is frequently questioned about what water level is right for a particular lake. This is often hard to answer, as the issues surrounding managing water levels can often more related to conflicts over use and other "social" issues than to environmental issues. However, there are a number of environmental questions often asked, and here are some thoughts about them.
Effects of water level management on a lake can be numerous. They include effects on water quality of the lake and the area downstream of the lake, on wetland conditions, and on plant and animal species and their habitats. Here "drawdowns" (significant, rapid reduction in water levels, usually for a limited duration) are distinguished from more usual water level management, which targets a narrow range of water level over the entire year.
- What is the effect of drawdown on a lake? Is there a beneficial effect on the water quality?
Some lakes are managed to reduce high phosphorus levels with significant (6 feet+) drawdowns, typically in September of each year. For such drawdowns to have a beneficial effect on water quality, the lake has to have:
- Very high phosphorus concentrations which peak at a specific time of year, typically late summer,
- The ability to drawdown the lake (dam capacity) sufficiently to export a large proportion of the lakes' water and thus much of its phosphorus in a short period of time, and
- The ability to rapidly re-fill the lake with relatively low phosphorus water.
Most lakes are not suited to benefit by this technique. Options usually considered by lake associations, towns and others who own dams are for variations of only a few feet at most.
- Are drawdowns an effective way to control shoreline erosion, siltation, and aquatic plant growth?
Drawdowns are usually not useful in controlling shoreline erosion unless they are the only way to prevent significant springtime ice damage, which aggravates the process of soil loss by root damage and scouring. Ice damage can often be reduced by attaining a stable, slightly lower water level in the fall and actively managing the dam to keep at the target water level through the winter season and into the spring. This level should be chosen to avoid ice contact with unstable or sensitive shoreline but not so low as to cause problems with wildlife habitat or uses such as water intake pipes.
Major level fluctuations can actually aggravate shoreline erosion in some areas if wave or ice action is concentrated below the level where cobbles and boulders have accumulated over the years. This is especially true for fairly steep shorelines where erosion below this cobble layer undercuts bank stability.
Increased siltation along a lake shoreline is often a result of some new sediment supply, usually relatively nearby. Siltation effects on plant growth are usually seen in coves and other protected areas and usually in less than 5-6 feet of water. Moderate amounts of shoreline erosion can promote growth of algae attached to rocks and plants, but usually this growth is more controlled by sunlight, amount of wave energy or disturbance, and the lakes' phosphorus and nitrogen levels. Drawdowns, which expose significant areas of fine sediment to drying, can result in release of phosphorus when the lake is re-filled and can expose previously stable sediments to wind and wave action, temporarily increasing turbidity and nutrient concentrations in the lake. Moderate water level variations which follow a long term "normal" pattern are not as likely to have as much negative effect in this regard.
Drawdowns have been used in some states to reduce dense populations of aquatic plants, especially nuisance exotic species, via desiccation and freezing damage. The track record is mixed as to the effectiveness, since some nuisance species can remain beyond the reach of the drawdown and effectiveness is usually limited in duration to a few years. Drawdowns can have significant negative impacts on native plant species that are critical parts of the lakes system, especially for the productivity of invertebrates and young fish. Again, lake plant communities exposed to moderate regular water level fluctuations usually adapt to this and form relatively stable growths. In addition, I often get reports of noticeable increases in plant density apparently not linked to water levels or siltation. Like land plants, populations of aquatic species can fluctuate due to natural causes, such as chance colonization by new species or vigorous individuals which increase plant density or as disease and other population factors cause them to decline. These processes are generally not well understood or predictable.
Although low levels of shoreline erosion may have little effect on the whole-lake water quality, it can have and effect on coves and limited areas by depositing fine material, which promotes plant growth. This is also caused when siltation occurs (especially in stream mouths and near temporary drainage-ways) as a result of upland erosion. Significant shoreline erosion, which is relatively uncommon on Maine lakes, can contribute substantial amounts of phosphorus. Regardless of the extent of shoreline problems, it is usually best to maintain water levels to reduce potential shoreline erosion. Finding that water level usually takes observation around the lake during all seasons to see how water levels affect various shoreline areas and in particular how water and ice would contact potentially unstable areas.
- How are fish, birds, amphibians, and warm blooded shoreline animals affected by water level changes? What species are especially at risk?
Significant fluctuations, especially drawdowns, can be highly disruptive to a number of animals. The effects depend on a several factors, not the least of which is the timing and duration of the change. Various fish species spawn from spring to fall. Some of them such as bass and minnows, spawn in spring or summer in the nearshore areas. Fall spawners such as brook trout favor springholes or stream mouths. Waterfowl nesting would make spring/early summer fluctuations problematic on many lakes, particularly those that have significant wetlands associated with them. Winter denning furbearers, especially muskrat and beaver, need stable water levels in late fall and during ice cover. Hibernating amphibians also are disrupted by mid-late fall and winter drawdowns. One other group particularly affected is freshwater mussels (a number of species of these are rare or endangered in Maine). Even the common species have a significant place in the lake ecosystem, filtering huge amounts of water and providing forage for ducks.
The integrity of wetlands and wetland plants can be threatened by significant, longer duration, changes in water levels. Even if a lake does not have large amounts of wetlands, this is one item to consider in water level management. Each lake is different and DEP and Inland Fisheries and Wildlife should review drawdown proposals for possible complications.
- Does a drawdown affect algae growth and concentrations?
As noted above, there are only a few lakes which will likely benefit from late season drawdown. Very large drops in water level during summer can change lake temperature stratification patterns and nutrient accumulations. Draining down the top several feet in a lake reduces the total lake volume by a very large amount and often exposes large areas of lake bottom. For example, the top three feet in many lakes can account for more than 10 % of the lakes' volume and bottom area. Again, each lake is very different depending on its shape and current water quality and it is often difficult to predict the results.
- How long does it usually take to re-establish a stable shoreline when the water level has been raised or lowered?
This is very hard to predict, and the results will vary not just between lakes but between different areas within a lake depending on such factors as:
- exposure to wave/ice action
- plant cover
- degree of shoreline disturbance
- soil type and supply of natural boulders in the eroding material
- presence/absence of cobbles and boulders at the base of the bank
- degree and timing of lake level change
Depending on the above, a stretch of shoreline can take many decades to re-establish a stable configuration after it begins eroding. This can be true even if there is no change in water level regime. Even lake shores which appear to be stable are often slowly eroding, and careful observation will reveal small areas of bank which are bare of plants or have exposed tree roots. This process has been significantly increased on many of the hundreds of lakes which have dams and extensive shoreline development or alterations even where these factors have been in place since early in this century. Unless it is obvious that water level management changes will reduce the frequency of water contact or ice damage to sensitive shoreline, it is usually best not to alter the management significantly.
- What can be done to stabilize shorelines?
It is advisable for landowners to stabilize eroding stretches with a combination of stone riprap and vegetation. Rip rap (which usually requires a DEP "Permit by Rule") is often highly effective and vegetation alone can often do a lot to reduce erosion. Each area of shoreline should be treated differently, as different plants and combinations will be needed depending on shade, slope, soil type and desired look the landowner wants. Generally, the same plants which do well as buffer plantings will do well in stabilizing sensitive nearshore areas, with the idea that deep rooted trees and shrubs are best. In general, we advise avoiding non-native species (use native plants, which you see growing around undisturbed shore nearby), plant a mixture or trees and hardy shrubs, and avoid reliance on perennial landscaping plants unless the owner is prepared to maintain the plantings. Advice can be obtained from County Soil and Water Conservation Districts or DEP. DEP will soon be publishing an updated plant list and other information. Also, local landscapers can often tailor a planting regime for a specific site, especially if they know the purpose is to stabilize the soil and banks.
In addition, property owners should take steps to alter factors which cause erosion and which channel water over the banks (vegetation can help here too). Things like establishing stable, winding paths, changing where they access the shore, and avoiding shoreline damage while seasonally moving boats and docks can help a lot.
- What is the best time of year for a drawdown?
Fish and wildlife considerations aside, timing of drawdowns depends on the purposes and competing uses, including such things as access for boats and docks, and water supplies to cottages. It is usually best to time drawdowns to allow rapid refill with fall rains before ice in. If moderate levels of drawdown are done for flood or ice damage control, it is best to target achieving a stable winter water level before ice in and strive to maintain that by active dam management.
In the end, one of the best ways to avoid problems with water level management is to have a clear plan for managing the dam which is built on a consensus of lake users and based on solid observation. This should include installation of a staff gauge and maintaining a regular log of water levels. Coupled with observations of water level effects and weather, this can allow dam owners to make informed decisions about what changes are needed over time and how weather patterns influence alter water input and levels. Without these records, it becomes difficult to sort out competing proposals for water level management.