Stratification Reduction

Greetings, I'm modeling a reservoir bounded by a dam on each end. The top part looks like a river, the bottom part (about 40 ft deep at the forebay) more like a reservoir. The field data shows good mixing and little to no stratification, even in the summer months. My model runs show pronounced stratification. I've tried smoothing the bathymetry so no cold water is trapped, increasing friction to induce mixing, eddy viscosity changes, etc to no avail. Any recommendations?? Thanks, Brett