Dam and river ecosystem basics module: introduction | hydrology | wetlands | morphology and sediment | water quality | biology | timing of dam repair or removal | references | appendix

Timing of dam repairs or removal

The timing of a dam repair or removal is critical for controlling sediment movement and performing restoration work, such as wetland restoration or revegetating the newly exposed land. Revegetation improves the appearance of the site, and it also helps with controlling sedimentation, provides habitat, and helps protect native plant species as they recolonize the area. It is important to understand that dam removal will result in temporary impacts on water quality, habitat, aesthetics, no matter what time of year it occurs. However, timing may be determined by the courts, not optimal biological timing. The following is a list of advantages and disadvantages for drawing down an impoundment and performing restoration work during each season of the year:

Winter

Advantages

• Frozen ground may improve accessibility to site.
• Leafless trees may improve visibility of site.
• Contractors may provide less expensive construction/deconstruction bids during the winter because it is their off-season.
• Low runoff during the winter months reduces erosion of exposed sediments.
• Winter snow will cover the exposed sediments.
• Construction/deconstruction activities may have lesser impacts on wildlife because little biological activity (nesting, spawning, migration) occurs during the winter.

Disadvantages

• Bad time if managing for brook trout due to ice tearing up redds, thermal effects, and sediment transport smothering the redds/eggs.
• Removal activities can adversely affect amphibians and reptiles that have buried their eggs or are hibernating in the sediments and along the stream banks.
• Some construction/deconstruction activities are more difficult to perform in winter.
• High snowmelt runoff in the spring may lead to increased erosion if sediment controls have not been installed.
• New vegetation will not appear until later in spring to help control erosion of sediment and improve the appearance of the exposed impoundment bed.

Spring

Advantages

• Vegetation not only improves the appearance of the site, but also helps with controlling sedimentation. New vegetation quickly establishes in the spring.
• It may be beneficial to drawdown the impoundment before waterfowl nest in early spring.

Disadvantages

• April through mid-June can be the most difficult period to receive inexpensive bids for a construction/deconstruction project.
• High snowmelt or rainstorm runoff may lead to increased erosion if vegetation or other sediment controls have not been established.
• The greatest biological activity (fish spawning, migration, nesting, egg laying) occurs during spring.

Summer

Advantages

• Summer is a common time for repair because the weather is favorable for construction activities.
• Vegetation not only improves the appearance of the site, but also helps with controlling sedimentation. New vegetation also quickly establishes in the summer.

Disadvantages

• High intensity rainstorms can cause high runoff and sediment erosion.
• Panfish spawn all summer, and thus will be affected by drawdown of the impoundment.
• During the summer, there is likely to be high public use of the water and greater visibility of the exposed sediments. Construction noise and dust may also be a nuisance to greater numbers of recreationists.

Fall

Advantages

• Fall is also a common time for repair because weather is favorable for construction activities.
• New vegetation can still be established in the early fall to improve the appearance of the site and control erosion of sediment.
• Leafless trees may improve visibility of site.

Disadvantages

• Fall rainstorms may produce high runoff that erodes exposed sediments.
• If work begins in late fall, new vegetation would not be established until the following spring to cover the former impoundment bed and provide sediment control.
• Drawdown of the impoundment in fall may impact waterfowl migrations.

 

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URL: http://www.ies.wisc.edu/research/wrm00/eductime.htm