The thermal landscape of a river system is shaped by a number of natural and human caused factors – solar exposure, air temperature, the volume and source of stream flow, and exchange between surface and subsurface water will all affect water temperature in streams and rivers. A critical question on the Middle Fork is to what degree have human actions altered these factors and how far has the system deviated from the range of pre-European settlement conditions under which Snoqualmie salmon and trout evolved (see conceptual model below).

The elevated water temperatures on the Middle Fork may be the result of natural factors such as the direction and width of the river valley or geothermal inputs. However, the Middle Fork may still be impacted by extensive 20^th century logging (see 1947 image of the Pratt River on this page) – while much of the riparian canopy has returned, stream channels may be wider after extensive logging due to increased sediment loads. Also, a reduction of large wood in the river may be limiting the number of pools that provide fish refuge and exchange between surface and subsurface waters. The changing climate may also be an important control – changes in air temperature and the volume and timing of runoff will have important impacts on stream temperature in the Middle Fork. Of particular concern are changes to the timing and volume of snowmelt runoff. Snowmelt provides a critical inflow of cool water during early summer but future climate models predict diminished mountain snowpack that will melt off earlier in the summer.