Current mean annual whole-lake total phosphorus (TP) concentration is about 20 mg·m−3, and mean June through September epilimnion TP is approximately 13 mg·m−3in Lake Sammamish. Water quality is referenced to mean June through September chlorophylla(chla) and Secchi disk transparency. Currendy, these two parameters are at approximately 2.8 mg·m−3and 4 meters, respectively. A dynamic deterministic TP simulation model indicates that lake water quality will decline, and future water quality in the lake is dependent upon die extent of land development and stormwater controls. The model simulated a lake that: 1) is seasonally stratified, 2) receives seasonal internal loading, and 3) receives seasonal variations in both external TP loading and hydraulic loading. TP movement into, out of, and within the lake was estimated based upon measured values for all of the major pathways. Past, present, and future water quality were estimated based upon the simulated TP concentrations and empirical relationships developed from both literature and in-lake relationships.