Re: South Platte River
The section of river between Spinney and Elevenmile reservoirs is tip-top wonderful, and there's some great spots closer to Hartsel, CO above Spinney as well. I can't recall how to find the spots where you can access the river above Spinney, and I DO RECOMMEND you get some advice on how to do so legally, but the info is definitely given out by shops in the area. The river there meanders through the meadow and the trout aren't huge, but there's plenty, and its a much more serene setting than the busier rivers in the area.
Keep in mind that in that area, you'll be at very high elevation and on a wide open plateau, so wind can be harsh, and weather can change at a moments notice. Even if it starts out at 70 degrees, you should have gear with you to keep you comfortable in sideways snow.