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Managing Greentree Reservoirs
By Phil Miller, Wildlife Biologist
One of the most underutilized practices for managing waterfowl is the artificial flooding of hardwood timber. These flooded areas, known as greentree reservoirs, are an excellent way to attract waterfowl.
Greentree reservoirs are manmade impoundments constructed of dikes or levees and water-control structures used to temporarily flood hardwood timber during the winter months. Dikes used to control the water level in the greentree reservoirs are relatively easy to construct using a bulldozer or a farm tractor and implements. Water levels can be raised or lowered at the appropriate time of year utilizing various structures, such as riser pipes, screw gates, or flash-board risers on culverts or drain pipes.
These reservoirs should be constructed in areas of bottomland forests with clay soils or soil types capable of holding enough water to flood three or more acres to a depth of at least 18 inches. These areas should have a prevalence of good mast-producing trees such as pin oak, willow oak, Nuttall oak, water oak and cherrybark oak. Undesirable trees can be thinned during the summer months to promote the growth of mast-producing trees and to create openings for waterfowl.
Flooding should occur about mid October or later when trees have become dormant. Flooding of trees during this stage will not harm the trees, but will increase their growth. Water control structures then can be shut off and water can be pumped in or rainfall can be used to flood the area. If water is needed to flood the area, small streams, wells or ponds are excellent sources of water. Water should be removed from the reservoir prior to the growing season or before the buds of trees begin to swell. This will prevent any damage, stress or death to the trees. If water control structures are working properly, the reservoir should drain in about one to two weeks.
Annual flooding of greentree reservoirs may, over time, promote the growth of undesirable flood tolerant tree species, such as baldcypress or water tupelo. To avoid this problem, flood greentree reservoirs every other year, or flood at different times during the late fall and early winter. During the spring, drawing the water down slowly in stages will slow the growth of undesirable tree species.
During the summer months, open areas within the greentree reservoir can be managed for native plants to benefit waterfowl or appropriate agricultural crops such as corn, browntop millet, foxtail millet, grain sorghum, smartweed, Japanese millet or soybeans can be planted around the edge.
If greentree reservoirs are lacking nesting cavities, place wood duck nesting boxes throughout the reservoir. These nesting boxes should be erected in December, prior to wood ducks searching for nest cavities in January.
For more information on constructing and managing greentree reservoirs, contact Phil Miller, Wildlife Biologist, Alabama Division of Wildlife and Freshwater Fisheries, P.O. Box 305, Northport, AL 35476.