Potential Management Strategies

The Wood River Wetland, and other drained wetlands adjacent to Agency and Upper Klamath Lakes, present difficult challenges to restoring wetland function, such as enhancing plant growth and retention of nutrients. The draining of wetlands has caused large losses of NH₄ and SRP to surface waters that produce exceedingly high concentrations of dissolved nutrients and salts. Such legacy effects require a water-level and water-quality management strategy that slows the oxidation of peat soils and reduces nutrient concentrations while promoting growth of wetland vegetation.

Water-Quality Management

Although the wetland generates large amounts of N and P, nutrient loading from the wetland to adjacent water bodies was eliminated during 2003–05 by not pumping water from the wetland (fig. 19). High water in 2006 increased the water-surface elevation to about 4,138 ft (NGVD29) in the North Unit (fig. 2), and water was pumped from the wetland to relieve stress on dikes and encourage seed germination of wetland plants (Andrew Hamilton, Bureau of Land Management, written commun., 2006). About 4,500 acre-ft of water was pumped into Wood River and Sevenmile Canal between April and early June 2006, lowering the water level by about 3.2 ft. Although the nutrient load may have been high, the biological effects to the receiving waters depend on many factors, including the rate of loading, dilution by receiving waters, and residence time. Management strategies that could reduce the negative effects of pumping nutrient rich water on receiving waters include varying the timing so potential nutrient loading would occur during the non-growing season (late autumn and winter), or pumping smaller amounts more frequently. Additional flushing of the wetland by flood irrigation and pumping may be required to dilute the sometimes exceptionally high levels of dissolved salts in the wetland surface waters that may interfere with seed germination, particularly in the South Unit.

Water-Level Management

Water levels influence biogeochemical reactions, seed germination, and other aspects of wetland ecology through their influence on habitat, DO concentrations, pH, temperature, and a number of other factors. The high DOC concentrations impart a brown color to the water that decreases light available for photosynthesis, and the high salt concentrations may interfere with osmotic water uptake in germinating seeds and young plants. Although adding water to the wetland during the summer could reduce the buildup of salts or DOC to levels more favorable for growth of wetland vegetation, lower water levels are required for seeds of many emergent plant species to germinate (Shipley and Parent, 1991). Bathymetric maps generated for the Wood River Wetland (fig. 3) can be used to optimize water levels in the North and South Units to help achieve management goals related to water quality and restoration of the wetland plant community.

To prevent further oxidation of peat soils, water could be added to the North Unit during summer to raise the water table and induce anaerobic conditions. Drying alters the structural integrity of soils, increases the surface area for decomposition, and allows air (with oxygen) to penetrate, further accelerating degradation. An intermediate water level may provide sufficient water to prevent oxidation of peat soils and would have the added benefit of increasing germination potential for wetland plants that cannot reproduce in a continuously flooded condition. Studies conducted on soil cores in the nearby Williamson River Delta Preserve detected higher nutrient losses from both flooded and dried cores compared with continuously moist cores (Aldous and others, 2005). Keeping soils moist and marginally flooded may be one management strategy to minimize nutrient loss in restored wetlands. Pumping of standing water from the wetland during spring to achieve water depths of between 25 and 55 cm (as determined during the Twitchell Island studies) might enhance plant growth, although creation of drier habitat may be necessary for initial seed germination. Plant growth can begin to rebuild peat soils, store carbon and nutrients, and reverse their losses.

One challenge for hydrologic restoration at the Wood River Wetland is that the land surface is sloped and irregular, with natural stream channels and deep canals that provide topographic relief. A particular water level will, therefore, produce a range in water depths throughout the wetland. The inundation analysis of the Wood River Wetland (fig. 20) portrays the number and size of ponded areas produced at a given water level that can be used to optimize water levels in the North and South Units.

Due to wetter than average precipitation during winter 2005, water was pumped from the wetland in April 2006 to ease pressure on the dikes. Future management could include permanent flooding to maintain saturated soil conditions and limit aerobic peat mineralization, with pumping to maintain target levels. Water level targets of 4,140.5 ft (NGVD29) in the North Unit and 4,135 ft in the South Unit were proposed by the BLM for enhancing seed germination (Bureau of Land Management, 2006). In the South Unit, this corresponds to the level where many small ponds begin to coalesce into larger water parcels (fig. 20) reaching a maximum number of ponds at about 4,134.8 ft (NGVD29). In the North Unit, the maximum number of ponds occurs at a water elevation of about 4,135.8 ft (NGVD29), which is 4.7 ft lower than the proposed target of 4,140.5 ft. This higher water level will result in fewer wetted parcels in the North Unit, but will produce a greater area of inundation, which will help prevent further oxidation of peat soils.

Many drained wetlands around Upper Klamath and Agency Lakes have subsided several feet, and reconnection with the lake can produce large areas of relatively deeper water habitat. In the Twitchell Island wetland, depths ranging from 25 to 55 cm produced conditions favorable for wetland plant vegetation propagation and growth, leading to accumulation of plant material (Miller and others, 2008). Future studies at the Wood River and other area wetlands might include similar experiments to determine which water levels are most favorable to optimize plant growth while minimizing decomposition rates and water losses from ET at greater depths.

In 2007, The Nature Conservancy wetland restoration project at the nearby Williamson River Delta Preserve removed parts of the dike and reconnected the former wetland to Agency Lake. Although this change may initially release DOC and nutrients (Aldous and others, 2005), over the long term, nutrient losses would be expected to decrease as vegetation becomes established and peat soils rebuild (Fleck and others, 2007). Regaining hydrologic connection to the lake will also produce a more natural flooding regime as wetland water levels decline in step with Upper Klamath and Agency Lakes, which drop an average of about 3 ft between June and October. Water may be applied to prevent soil drying during the late summer and early autumn, when water levels are lowest.

Capping Artesian Wells

Although the five artesian wells discharge 1 percent of the total water inflow, the water has relatively high nutrient concentrations (about 6,000 µg/L of NH₄-N and SRP), much higher than those measured in Agency and Upper Klamath Lakes. Stopping the flow by decommissioning or capping these wells would reduce this input of nutrients to the wetland.