Northeast of Denver near Estes Park, the Big Thompson River (aka Big T) is a surprising large river for the Front Range. The upper Big Thompson River starts near Moraine Park in the Rocky Mountain National Park and flows east through Estes Park and down into Lake Estes.
Above Estes Park, the upper Big Thompson tends up offer up trout up to 14 inches long. The caddis hatch is typically June through September on this stretch.
Note: The printable fly fishing river map is provided courtesy of HANDYMICROFIBERCLOTH.COM.
Overall, Big Thompson River - Upper is the 9th most popular fly fishing river of all 47 fly fishing spots in Colorado.