I was recently made aware of this post (hat tip to Larry and Karl)
It describes the impact of heat upon the rivers and oceans. I was struck by Figure 2:
The amount of energy that reaches the ocean from the Connecticut River, shown in dark blue, is comparable to the electrical output, although another graph said that the amount of heat lost to the river, light blue, could vary from 20% in the summer to 39% in the winter.
Figure 3 showed a map showing how extensive the thermal plume is:
I did not see any mention of the impact of freezing and ice on the model. Snow on a frozen river reflects most of the solar heat. Once the river has changed from ice to water at the reactor, the reflectivity of the surface of the river changes from an albedo of around 60, ice and snow, to that of water, about 6. Once the river has melted, the sunlight is absorbed rather than reflected, and more heat is added to the river. We note this in the spring, when the snow finally melts, and the earth can get warmer faster when the earth is finally brown. As the there was more melting, there is more area to absorb the heat, creating a positive feedback loop. This effect would extend further how far the river was open before it froze again. 
Light is reflected by fresh snow and absorbed by water. Once the open water is formed, the sun will further warm the water.
Vernon Radiation Safety 