■ Watersheds, Geology, 

 and Climate 



Stream channels are formed by the flow of 

 water and the load of sediment they carry. 

 The amowit of water and sediment moving 

 through a watershed depends on climate 

 and geology. 



Climate determines: 



• amount of rain and snow, and the 

 timing of runoff 



• rate of evaporation 



• type of vegetation 



• rate of groundwater recharge 



• rate of erosion 



• how quickly bedrock weathers to soil 



Geology determines: 



• rate of erosion 



• potential for sedimentation 



• stream bank material 



• permeability and porosity of soils 



• aquifer depth and recharge rate 



• water chemistry and biological 

 productivity 



Precipitation in Montana falls 

 unevenly — most falls in the western 

 third of the state, west of the 

 Continental Divide. Because of this, 

 more water flows out of the state in the 

 Clark Fork River than in the Missouri 

 River, which collects water from the 

 eastern two-thirds of the state. 



Average Annual Precipitation 



Peak flow, the greatest flow in a stream, typically 

 occurs in western Montana during late spring and 

 early summer when snow melts in the mountains. In 

 eastern Montana, some streams experience high flows 

 after intense rainstorms in spring and summer. 



Base flow, the lowest flow in a stream, typically 

 occurs in western Montana from late summer through 

 the winter. Flows may go entirely below ground 

 during dry periods. 



Average Annual Runoff 



Width of stream tine corresponds to top width of channel. 

 IVIean annual discharge, in thousands of cubic feet per 

 second, Is represented by channel cross section in blue. 



10 



^ ^ ^ 



20 30 40 



Less than 14 inches 

 14 to 30 inches 

 More than 30 inches 



Forested, mountainous watersheds 

 in northwestern Montana receive 60 

 to 100 inches of precipitation per 

 year and have more perennial 

 streams; grassy plains watersheds of 

 eastern Montana, which receive 8 to 

 12 inches of precipitation, have more 

 intermittent and ephemeral streams. 



