retired NC Driking Water Treatment guy here. everyone is right! the stratification aspect does play a very important role, but the water isn’t always stratified, especially this particular time of year in many reservoirs. in fact, late winter/early spring always gave us fits. cold, dense, muddy (turbid) source water is very difficult to treat (floc out). most raw water reservoir intakes are designed with the ability to “draw” water from two, or more, depths, in order to minimize the effects of various raw water quality issues brought on by summer stratification and fall turnover. but the cold and muddy water in late winter/early spring always seemed to find it’s way into the intake no matter what depth we drew from. it was unavoidable. riverine systems will obviously be more prone to a thorough “mixing” too.