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 simpler (depending on the material) and questions related to material budgets are often related in terms
of material loading to the lake. 'The term 'loading' correctly implies that there is an accumulation of
some material of interest that is being transported into the lake from some external source, most often
streams or seepages. In most reservoirs, loading is a concern because reservoirs are often located in
watersheds having activity which contributes allochthonous materials of interest to the inflowing streams.
Among these materials are sediments, organic materials, toxic materials, and the nutrients. Only the
nutrients will be discussed in this workshop.
Reservoirs are very sensitive to loading because of the proportionally larger watersheds
compared to natural lakes. The greater watershed area and resulting inflows of materials are often
trapped by reservoirs. Figure 1.2.8 illustrates the effect of construction of Richard B. Russell Lake on
phosphorus loading to J. Strom Thurmond Lake, another reservoir just downstream. The phosphorus
loading by the Savannah River prior to Richard B. Russell Lake was greatly decreased as a result of the
impoundment of Richard B. Russell Lake. Another major tributary to J. Strom Thurmond Lake is now
the major source of phosphorus to that lake although the amount of water entering from the Savannah
River has not decreased. Furthermore, because much of the phosphorus leaving Richard B. Russell
Lake is dissolved phosphorus, the form of this nutrient loading has been changed as well.
The two variables that must be measured to assess inputs of materials are streamflow and
concentration. These may then be used to compute actual quantities of materials entering an aquatic
system. Concentrations may be measured chemically, flows are measured using some type of gage. The
time-dependent record of flow for a stream is termed a hydrograph. Figure 1.2.9 depicts a theoretical
hydrograph showing base flow and storm flow for a hypothetical
1.2-19
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