A mathematical model of Lake Ashtabula Reservoir, Valley City, N. Dak., was developed based on the one-dimensional reservoir thermal model, CE-THERM-R1. The major assumption employed in CE-THERM-R1 is that the reservoir is represented as a vertical series of completely mixed horizontal layers (one-dimensional assumption). Longitudinal and lateral variations of constituent concentrations cannot be predicted. Lake Ashtabula, whose pool is long, narrow and largely confined to the run-of-the-river, exhibits both vertical and longitudinal water quality gradients. To represent this two-dimensional aspect of the system, the model was organized using three CE-THERM-R1 pools coupled in series in which the simulated outflow constituent quantities for the first and second pools became the inflows for their respective downstream pools. By using this model configuration it was possible to simulate longitudinal as well as vertical temperature variation and the concentration of total dissolved solids, thus reducing the severity of the one-dimensional assumption.