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Hydrologic Study of Table Rock Lake
Table Rock Lake, located in southwest Missouri, is a large hydropower reservoir with hypolimnetic discharge. In summer, nutrient-poor hypolimnetic outflow from Beaver Lake flows as a density current into the hypolimnion of the White River arm of Table Rock Lake with little effect on surface water quality. Nutrient-rich inflow from other tributaries enters the epilimnion, resulting in a measurable increase in algal biomass and reduced water clarity. Dilution and sedimentation of these inflows cause extreme longitudinal gradients and downlake declines of >80% in the levels of phosphorus and chlorophyll have been recorded in the James River and Long Creek arms of the reservoir. Local nutrient inputs from Leatherwood Creek and other small inflows, cause similar, but smaller gradients.
Recent data show that in some years thermal structure in Table Rock is such that these nutrient-rich inflows plunge as a density current into the hypolimnion without much direct impact on water quality in the epilimnion. When nutrient-rich inflows enter the lake as density currents water quality is better than might be expected on the basis of nutrient loading. It is suspected that large density currents were important in the apparent improvement in water quality in 1996 as compared to other years in the recent past. Simply put, the effect of external nutrient loading on in-lake measurements of water quality in Table Rock Lake is complicated by complex hydrological patterns. The extent that nutrient-rich inflows enter the epilimnion relative to periods when they enter the lake as density currents will directly effect water quality within the highly visible epilimnion.
A better understanding of this process is needed to quantify the benefits of phosphorus removal efforts within the basin. It seems the benefits of phosphorus removal will be greater in years when inflows enter the lake as density currents and nutrient loading is directed deep into the water column. In contrast, these benefits will be less apparent when inflows enter the epilimnion directly, thereby stimulating algal growth and causing a decline in water clarity. Quantifying the role of density currents on lake water quality is essential to interpreting the benefits of phosphorus control to the lake users and taxpayers.
PROJECT DESCRIPTION
The project approach to this question is two-fold. First, it has been shown that mean summer Secchi transparency during 1976-1998 regressed on total water year inflow (ha.m) shows a strong relationship with total inflow. This follows general limnological theory in that increased nutrient loading from increases in total water inflow leads to greater algal biomass and decreased transparency in lakes. Data from several years within the period of record, however, fall outside the general pattern and it is suspected that these are years when density currents were a factor. Dr. Mark Kaiser, Statistics at Iowa State, has offered to evaluate this multi-year data set. He will determine if we can link variation in the water quality-inflow pattern to features of climate (ambient temperature and the onset of thermal stratification relative to inflow events) or hydrology (magnitude and timing of inflows, withdrawal of water from the hypolimnion, or lake level fluctuations). He has extensive experience with lake modeling.
Second, water masses will be traced in the major arms of Table Rock seasonally to determine how inflows are integrated into Table Rock. This will be done by using conductance, chloride and calcium concentrations as conservative measures of specific water masses entering the lake. Nitrate and soluble reactive phosphorus will also be measured to determine the loading of available forms of these key elements and uptake patterns during passage through the water body.
Samples will be collected along a longitudinal transect of the major arms of the reservoir; this includes the James, Kings, and White arms. Samples will be collected at sites along a longitudinal gradient moving down the lake from the major input to the reservoir. The distance of each longitudinal profile will be determined by the results of our field sampling. Initially sampling sites will be located at intervals of two miles and this distance will be reduced in zones of change (such as dilution or plunging water masses).
At each site measurements of oxygen, temperature and conductivity will be collected at one or two meter intervals throughout the water column. Samples will be collected from various depths for subsequent chemical determination in the laboratory. Sampling of these inflow patterns will occur prior to thermal stratification (in early-March), and after stratification is established (early summer). In addition, during periods of high inflow (both during the stratified period and when the lake is homothermal) we will trace inflows of both large and smaller streams to determine movements within the reservoir. These data will be used as case studies to interpret the multi-year analysis of the role of density inflow. Each year we anticipate sampling inflow patterns on four occasions and these sampling trips will not coincide with our routine water quality monitoring.
This project complements the Table Rock Lake Long-term Monitoring Project also funded through Section 319 at this time.
OBJECTIVES
Quantify the role of density currents on lake water quality.
PRODUCTS
A final report on this assessment will be the only product.
PROJECT SPONSOR
University of Missouri-Columbia
COOPERATING AGENCIES
University of Missouri-Columbia
Missouri Department of Natural Resources
CONTACT
University of Missouri
302 Anheuser-Busch Natural Resources Bldg.
Columbia, MO 65211-7240
Dr. J.R. Jones 573-882-3543
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