Excavation of a deeper channel in Lake Ogallala improved conditions for fish in the lake by increasing dissolved oxygen and vegetation density. This work was accomplished through a multi-agency partnership and significantly under original cost estimates.
By Mark M. Peyton and Jeffrey J. Buettner
Lake Ogallala in western Nebraska began as a borrow pit for the construction of Kingsley Dam but today has a reputation as an excellent cold-water fishery.
The Central Nebraska Public Power and Irrigation District built Kingsley Dam between 1936 and 1941 to form Lake McConaughy, the storage reservoir for the district’s irrigation and 51.9-MW Kingsley hydroelectric project. During construction of the dam on the North Platte River, huge electric dredges – one on each side of the dam’s centerline – pumped sand and gravel from the riverbed to form the upstream and downstream embankments around a water-tight clay core. The two dredges removed more than 19.8 million cubic meters of sand and gravel from the riverbed in building the 5.6-kilometer-long dam. Lake Ogallala was formed by the dredging activity on the downstream side of Kingsley Dam.
Almost from the time it filled with water, Lake Ogallala became known as an excellent lake for fishing. Water that enters Lake Ogallala comes from the bottom of Lake McConaughy, resulting in water temperatures that rarely exceed 18 degrees Celsius. These are perfect conditions for rainbow trout.
The lake covers 260 surface hectares and is divided into two basins. The north-south basin, created during the dredging process, has an average depth of about 10 meters. The east-west portion, known as the Keystone Basin, overlies the old river channel and is less than 3 meters deep in many places. Keystone Dam, a diversion dam constructed a few years before Kingsley Dam, forms the eastern boundary of Lake Ogallala.
This lake is used to regulate water flows for downstream power and irrigation projects. But its status as Nebraska’s premier year-round trout fishery prompted efforts to address issues related to dissolved oxygen (DO) concentrations in the lake.
Declining DO levels
Before the Kingsley plant was built in the early 1980s, releases from Lake McConaughy passed through an outlet tower. The tower’s gates lie at the bottom of Lake McConaughy, where DO in the water is absent or minimal during the summer months. The water flowed through a steel-reinforced concrete penstock into Lake Ogallala. Upon exiting the penstock, the water passed over a series of concrete baffles on an apron in the stilling basin that were intended to inhibit erosion in the basin but also introduced oxygen to the water.
The baffles were removed when the hydroelectric plant was constructed in this stilling basin. The penstock from the outlet tower was converted to serve as the conduit to the hydro plant, and the water remains virtually devoid of oxygen as it enters Lake Ogallala. In addition, Lake McConaughy has experienced continued eutrophication (the proliferation of plant life, especially algae, which reduces the DO content in the water), resulting in a general decline in DO concentrations in Lake Ogallala.
Studies performed by the Central Nebraska Public Power and Irrigation District in 1986 and 1987 indicated that partially opening the hydro plant’s bypass valve would aerate the anoxic water coming from Lake McConaughy and raise DO to a level sufficient to support the trout fishery in Lake Ogallala. This strategy has been employed since 1988.
However, use of the bypass valve for aeration purposes reduced electrical generation at the hydroelectric plant by an average of 20 percent during the summer months (June through September). In addition, the bypass valve was not designed for such frequent use, and the abnormal wear and tear required costly disassembly, repair and reassembly during the winter of 1995-1996.
Then, in 1998 and 1999, fish kills in Lake Ogallala that were associated with areas of low DO indicated that use of the bypass valve alone was insufficient to overcome low oxygen conditions in all areas of the lake during all times of the year. To help solve the problem of low DO areas in the deeper portion of the lake, the Nebraska Game and Parks Commission, in cooperation with the Central Nebraska Public Power and Irrigation District, placed aeration systems in two locations within the north-south basin. These systems consisted of compressed air piped to a web of manifolds on the floor of Lake Ogallala. The systems raised DO in those areas of the lake, but they also created surface-to-bottom circulation patterns that resulted in warmer, less attractive water for trout.
Subsequently, the Nebraska Game and Parks Commission replaced the compressed air system with a low-volume pure oxygen system in 2003. Although this eliminated the top-to-bottom circulation that led to the warmer water, the pure oxygen system was inadequately designed for the size of the lake and required frequent maintenance because of repeated breakdowns. It also was insufficient at measurably raising the level of DO.
The inadequacies of the aeration systems and the fish kills led to the development of a cooperative study into the causes of low oxygen conditions within areas of Lake Ogallala during late summer. The study was conducted by Central Nebraska Public Power, the Nebraska Department of Environmental Quality, Nebraska Public Power District, Nebraska Game and Parks Commission and University of Nebraska-Lincoln.
Extensive sampling and testing were conducted in September 2000. Based upon the results of those studies and water quality monitoring conducted throughout the summers of 2000 and 2001, in 2002 the Nebraska Department of Environmental Quality added Lake Ogallala to the Clean Water Act Section 303-d list of impaired water bodies because of concerns regarding DO levels in the lake. Subsequent water quality studies identified sulfates originating in Lake McConaughy and the North Platte River Basin, circulation problems within Lake Ogallala (specifically in the Keystone Basin) and the “fill and release” operational cycle as causes for low DO levels in specific areas of Lake Ogallala.
The parties entered into a cooperative agreement to develop a plan to overcome the low DO areas within Lake Ogallala. The group, with partial funding from the Environmental Protection Agency, contracted with the University of Nebraska-Lincoln Engineering Department to develop and test various alternatives for addressing the problem of low DO in Lake Ogallala.
University personnel constructed a large physical model of the lake to determine if changes in its topography could help eliminate low oxygen areas. In association with the physical model, multiple computer models, including CE-QUAL-W2, were constructed to test various alternatives.
The Nebraska Department of Environmental Quality completed a total maximum daily load assessment for Lake Ogallala in September 2007. As part of this process, a plan of action to address the various problems identified in the assessment was developed. The EPA approved the assessment in September 2007.
Channel excavation
A plan was developed to change the circulation pattern in the Keystone arm of Lake Ogallala, based upon a list of alternatives provided by the University of Nebraska-Lincoln engineering department.1 The chosen alternative required cutting a channel 3 to 4 meters deep through a wetland that partially separated the north-south basin and the Keystone Basin. Once through the wetland, the channel would continue along the northwest side of the basin.
The purpose of the channel was to provide a deeper water refuge for fish along the north side of the shallow Keystone Basin, reduce the size of the stagnant area in the northwest corner of the basin by increasing flow through the north reach of the basin, and provide a pathway for the fish from the shallow water areas to the deeper north-south basin.
This plan called for the removal of about 61,775 cubic meters of material in a wetland. The project began in October 2009. The channel through about 300 meters of wetland was cut 2 to 2.3 meters deeper than the surrounding lake bed, with a 15-meter-wide bottom and a 4:1 slope.
East of the wetland is the large, flat and shallow Keystone Basin. The channel through the wetland continued for 1,576 meters through this shallow portion of the lake. The depth of this segment of the channel ranged from 1 to 2.3 meters deeper than the lakebed. Creating the channel required the excavation of 89,127 cubic meters of material.
It was not feasible to remove the spoil from the excavation to an upland site, so it had to be piled on the adjacent wetland. The U.S. Army Corps of Engineers determined that the project would impact about 2.13 hectares of wetland and therefore required mitigation of 4.26 hectares. Spoil taken from the 1,576 meters of lake bed was used to create a 5.26-hectare wetland within the Keystone Basin.
Three major hurdles were encountered during excavation of this channel. The first was the limited window of time in which to accomplish construction. Water from Lake McConaughy is passed through Lake Ogallala to supplement supplies in the Nebraska Public Power District’s Sutherland Reservoir for cooling purposes at a 1,350-MW coal-fired plant. Sutherland Reservoir has a sufficient volume of water to cool the plant without supplemental water for only about 60 days during the autumn months. Thus, the district expected that the project would require two phases in successive autumns to complete.
The second hurdle was the fact that multiple projects were under way while Lake Ogallala was drawn down. The Nebraska Public Power District was replacing three control gates on its Keystone Canal, which takes water from Lake Ogallala to Sutherland Reservoir. At the same time, the Nebraska Game and Parks Commission took advantage of the low volume of water remaining in Lake Ogallala to renovate the fishery. The chemical rotenone was placed in the water to kill the lake’s fish population and, for the next four weeks, water was not allowed to move from the lake down the North Platte River. Seepage water raised the lake level about 1.5 meters over the four-week period, inundating the lower portion of the channel in the Keystone Basin.
The final hurdle was the weather. The largest quantity of snow ever recorded in a one-month period in the area (77 centimeters) fell during October 2009. The snow, along with above average rainfall and increasing lake elevation, resulted in significant difficulties with mud that required multiple movements of the excavated material.
Results
Within the 60-day window given for the project, Central Nebraska Public Power and Irrigation District personnel excavated 1,879 linear meters of channel with a bottom at elevation 952.6 meters above mean sea level (2.4 meters below the operational level of the lake at low pool), 15 meters wide with a 4:1 slope. About 300 meters of excavation through a wetland resulted in a 2.13-hectare impact, requiring the construction of a minimum of 4.26 hectares of mitigation. More than 5.2 hectares of new wetlands were constructed.
The results of the change in the bathymetry of the lake on DO and the subsequent impact on the fishery, as well as the development of the mitigation wetlands, will be evaluated over the next five years.
The Nebraska Game and Parks Commission monitors fish survival, growth, health and harvest through fish sampling and angler creel surveys. The Nebraska and Central Nebraska public power districts monitor aquatic plant growth in the shallow area of Keystone Basin, and Central monitors DO at 21 locations within the basin. In addition, Central monitors the development and species composition of vegetation within the mitigation wetland. And the Nebraska Department of Environmental Quality and University of Nebraska-Lincoln monitor overall water quality.
After one year, the results are promising. The DO content of water in the dredged channel was consistently higher on four monitoring dates than in the adjacent, shallow water area (see Figure 1).
Over all sites and all monitoring events, the DO level in the channel averaged 1 part per million (ppm) more than the adjacent sites. Only one of 20 samples from adjacent sites was greater than the corresponding sample from the channel (August 13), and the DO content within the project area dropped below 3 ppm on only one occasion (August 27). DO dropped below 3 ppm at nine sample locations outside the project area on that same date.
The aquatic plant growth in the Keystone Basin of the lake is strongly correlated with the size of the “rough” fish (common carp and white suckers) population in Lake Ogallala. After the renovation of Lake Ogallala in 1997 and the removal of the rough fish, aquatic vegetation in the shallow Keystone Basin of the lake responded for a few years until the population of the herbivorous rough fish once again reached levels that made growth and survival of the vegetation impossible (see Figure 2).
The pattern present from 1998 to 2001 appears to be repeating itself after the 2009 lake renovation. Vegetation was non-existent in the Keystone Basin during the summer of 2009 preceding the project and associated lake renovation by the Nebraska Game and Parks Commission. By contrast, the density of vegetation surpassed 3,700 grams of plant mass per square meter in 2010 (see Figure 3).
The fish population’s positive response to the improved water quality is evident, as is the increase in aquatic vegetation and the invertebrate population associated with that vegetation. While creel data from the lake has not yet been completely analyzed, preliminary results indicate that fish stocked in the lake after the project’s completion are healthy and exhibit excellent growth rates, some exceeding 1 inch per month. Many trout from the initial stocking in late December 2009 have exceeded 20 inches in length.
The Lake Ogallala Bathymetric Improvement Project was made possible by the significant time and effort invested by a number of individuals, agencies and organizations. Throughout a long and challenging process that included monitoring, research, development of both computer and physical models, and model evaluation, the project’s cooperators were able to develop a cost-effective way to address the problem presented by areas of low DO in the Keystone Basin of Lake Ogallala. The cooperation between the many agencies – all with different priorities and interests – was a key component in the development of the project, says Dave Tunink, assistant administrator of the Nebraska Game and Parks Commission’s Fisheries Management Section:
“The Lake Ogallala Bathymetry Improvement Project was accomplished after many years of field investigation, laboratory testing and numerous multi-agency discussions to determine the most cost effective methods to be used to address water quality issues. The project just completed will improve the water circulation pattern of this portion of the reservoir along with providing trout an area to escape the shallow east portion of the Keystone basin via the newly created channel to the South basin. Since Lake Ogallala has a long-term database of both fish population and water quality, the impact of this project will be able to be evaluated in great detail. This project demonstrates how various agencies can work together to develop, implement and complete an environmental enhancement project which will benefit the recreational users for many years.”
Funding was made available from the EPA’s Section 319 Nonpoint Source Control Program administered through the Nebraska Department of Environmental Quality, as well as the Nebraska Environmental Trust Fund. In-kind and other financial support was provided by the Central Nebraska and Nebraska public power districts and Nebraska Game and Parks Commission. Projected costs – based upon a two-year completion time – were $815,000. However, because the Central Nebraska Public Power and Irrigation District was able to do the work in-house and completed the project in a single year, the actual cost of this project was $316,000.
Note
1Admiraal, David, et al, “A Physical Model Study of Flow Patterns in Lake Ogallala,” University of Nebraska-Lincoln, 2003.
Mark Peyton is senior biologist and Jeff Buettner is information and communications officer for The Central Nebraska Public Power and Irrigation District. Peyton was one of the coordinators of the bathymetry improvement project.
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