The Clackamas river is affected primarily by the amount of rainfall that has occurred within the last few days. This can make for some prime fishing during the fall, winter and spring months. However, depending on the amount of rain, the river can be "blown out", or too high and discolored to fish. 

The Sandy river is primarily influenced by what happens on Mt Hood. In the spring and summer when the snowpack on Mt Hood is melting, the river turns "glacial", or a milky green color. In some cases this can be to the angler's advantage; in other cases, it can make finding the fish somewhat difficult. During the fall and winter, the river conditions are largely dependent on the snow level. If the snow level is low (2,000 to 4,000 feet), a heavy downpour in the Portland area will not affect the river level as much as a lighter rain when the snow level is higher.

The Nestucca, Wilson and Trask are all coastal rivers that are influenced by the Oregon coastal mountain range. In general, the Oregon coastal range doesn't accumulate snow the way Mt Hood will. The coastal mountain range is relatively low in comparison and while it will accumulate snow under certain conditions, it tends to melt off quickly. Overall, these coastal rivers clear quickly after rains and make for some excellent fishing when the conditions are right; this also means that conditions on these rivers can be "just right" for only a day or two during the rainy late fall, winter and early spring times.