The Eau Pleine Reservoir has historically experienced point and non-point source (NPS) pollution resulting in winter anoxic conditions and periodic fishkills. Because the reservoir has the potential to support a quality sport fishery, including walleye (Stizostedion vitreum) and northern pike (Esox lucius), minimizing the incidence of anoxic winter conditions is a management goal. During winter reservoir drawdown, a dissolved oxygen (DO) sag typically develops in the headwater and moves through the reservoir, leading to periodic fishkills. To identify factors that influence the DO sag and to determine if those factors could be managed, I evaluated historical fishkill records, winter DO data, river base flow (inflow), precipitation/runoff events, and reservoir operations data collected over a 24-year period. A combination of three key factors consistently affected the character (magnitude and duration) of the DO sag and the incidence of fishkills. No fishkills occurred when 1) reservoir volume at the start of winter drawdown was greater than 60%; 2) river base flow was 6 cfs or greater; and 3) the DO sag did not develop until after February 1.1 postulated that anoxic conditions during winter could be minimized by using these factors as guidelines for future reservoir operation, along with continued use of two aeration systems. Reservoir operating guidelines were developed and have been followed since 1993; no fishkills have occurred.