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4. RESTORATION ACTIONS

4.1. Dredge and Grade Pond Bottom and Construct New Outlet.

In my considered opinion, the single most effective set of actions that can be taken to achieve the Thurston Pond ecosystem restoration goals would be to dredge and grade portions of the pond bottom and to construct a new pond outlet structure. Collectively these actions would eliminate the unattractive mud flat, provide more deepwater habitat, and permit the pond to be drained when needed to improve pond water and pond bottom sediment quality and to manage pond plant and animal communities.

A minimum dredging and grading effort that would provide more deepwater habitat and allow the pond to be fully drained is shown in Figures 2 and 3. The dredged channel extending from the northeast corner of the pond to the pond outlet on the southwest corner of the pond should be graded to allow water to flow by gravity from the deepest portion of the pond to the pond outlet. The existing pond outlet consists of a grated overflow outlet (Figure 4) that drains into an underground tile system (Figure 5), which connects to the storm sewer running parallel to Prairie Street and discharging into Miller Creek near Plymouth Road.

The elevation of the bottom of the underground tile at the overflow outlet is about 4 feet below the overflow grate. Thus, completely draining the pond after dredging and grading as outlined above would require either (1) lowering the underground tile at the overflow outlet by about 3 feet, or (2) first draining by gravity flow, followed by pumping the residual water out of the pond. Digging a settling basin at the pond outlet (Figure 5) would facilitate pump-out of residual water and allow the soft sediment that migrates to the outlet during pond dewatering to be removed by pumping for on-land disposal in the TNC.

4.2. Increase Island Elevation and Add Offshore Diked Wetland.

I recommend that a portion of the dredged pond-bottom soil be used on site to increase the elevation of the existing island (Figures 6, 7,and 8) and to create a new offshore diked-wetland (Figure 9) that is either adjacent to or contiguous with the island. The island should be capped with loose sand to provide nesting habitat for turtles. Water depth inside the diked wetland should be shallow enough so that it will freeze to the bottom over winter, thus providing fish-free breeding and nursery habitat for amphibians in spring and summer.

4.3. Increase Berm Elevation.

I further recommend that the remaining portion of the soil dredged from the pond bottom be used on site to elevate the berm at the west end of the pond (Figure 3) by about 1-2 feet so that the historical maximum water level in the pond can be reestablished. The maximum water level now attainable in the pond is about 6-8 inches lower than the historical maximum. This historical maximum was established in the 1970s when the berm and the existing overflow outlet structure (Figure 4) were constructed at the west end of the pond to maintain pond water levels and manage the pond outflow. Since then the berm has sunk to the point that during periods of high rainfall and snowmelt, water flows over the berm instead of through the overflow outlet. We recognize that water levels will naturally vary seasonally and from year to year and that such changes are appropriate and will help achieve other goals, such as enhancing habitat diversity. However, we recommend that the berm be elevated and reconfigured as needed to reestablish the historical maximum water level in the pond, against which the natural seasonal and annual fluctuations can occur.

Water temperature in the pond typically ranges from about 20 to 30^°C in mid-summer (Figure 10) to 0^°C in winter during the period of ice cover and provides thermal habitat that can support a warmwater aquatic community typical of the region. However, much of the pond is so shallow (Figure 2) that the pond water in these areas freezes solidly to the pond bottom over winter. Reestablishing the historical maximum water level together with deepening portions of the pond by dredging will help ensure that significantly greater amounts of deepwater habitat remain liquid during the period of ice cover, thus favoring the survival of a healthy, warmwater aquatic community.