ABSTRACTWater quality data on six hypereutrophic urban lakes in Baton Rouge, La., have been collected on a regular basis over a 4-year period as part of a cooperative restoration effort by the U.S. Environmental Protection Agency and the city/parish of East Baton Rouge through the Clean Lakes Program. Thirteen water quality parameters were measured during most of the sampling events, including total phosphorus (TP) and dissolved oxygen (DO). Concentrations for TP and DO during the pre-restoration period ranged from 0.136 mg/I to 1.340 mg/I and 0.40 mg/I to 25.30 mg/I, respectively. This observed variability during pre-restoration illustrates the instability present with these systems. Inherent with this instability is the occurrence of water quality problems, such as summer fishkills. A probabilistic approach was developed to describe the inherent variability typical of these systems. The use of a probability density function to estimate overall summer kill risk is also discussed. This technique was applied to one of the smaller lakes in the system to evaluate restoration efforts. Overall projected fishkill frequency was reduced from approximately eight kills per year to one kill in 7 years following restoration. Such approaches can be used in management to evaluate data more readily where standard procedures are time consuming and funds unavailable.