Climate 



Climatic factors are major determinants of the physical and biological 

 characteristics of streams through their control of air and water temperature 

 and the amount and pattern of precipitation. 



Temperature . Air temperature has a direct effect on the freeze-up and 

 break-up of streams. Freezing may occur in two ways: surface ice forms when 

 air temperature drops to freezing levels at the air-water interface; anchor 

 bottom ice is formed when climatic conditions (i.e., cool temperatures and 

 clear skies) allow radiational cooling of rocks, logs, or other immersed 

 objects to the extent that ice forms around them. Surface ice interferes with 

 the exchange of gases between air and water, and reduces the amount of light 

 penetrating the surface (especially if snow covers the ice). Anchor ice 

 around objects prevents organisms from attaching to them and may trap 

 organisms, causing deaths. Scouring of the substrate may occur when anchor 

 ice breaks up, resulting in dislodgment of organisms and disturbance of 

 sediments . 



Air temperature, in conjunction with light-intensity, tree-canopy cover and 

 current velocity, also influences year-round water temperatures. Water 

 temperature has a major influence on the rates of production, respiration, and 

 decomposition of stream organisms; thereby affecting nutrient and energy 

 cycling within the stream system. Temperature also affects the distribution 

 of stream organisms. For example, in Maine streams brook trout and other 

 salmonids require cool temperatures (65°F; 11°C) , while smallmouth bass grow 

 and reproduce best under warmer conditions. 



Air temperature affects stream conditions indirectly, also, through its 

 influence on evapotranspiration (loss of water by direct evaporation at the 

 surface and through plant use). Evapotranspiration is a factor determining 

 the percentage of entering precipitation that remains in streams. High 

 temperatures increase evapotranspiration rates. 



The southwest to northeast air temperature gradient along the Maine coast 

 results in warmer water temperatures and earlier ice breakup in southern 

 areas, cooler temperatures and later ice breakup in northern areas 

 (Lautzenheiser 1972; Fobes 1974; and Davis 1966). 



Precipitation and flow levels . Water in streams may come from direct 

 precipitation, surface runoff, through-flow (lateral movement of water through 

 the soil) or ground-water discharge. The primary source of all these waters 

 is atmospheric precipitation. Therefore, the amount and pattern of 

 precipitation in an area are the ultimate determinants of the amount (i.e., 

 flow) and pattern of stream flow in that area. 



Annual precipitation in Maine (see "Climate," page 2-9in chapter 2) is greater 

 than that in much of the United States and is sufficient to maintain 

 comparatively large rivers with moderate-sized drainage basins. 



The flow pattern of a river is frequently equally or more important than the 



amount of flow in relation to physical and biological processes. For example, 



most channel-cutting erosion occurs during peak flow periods, and a stream 



with a very irregular pattern of flow may erode its channel much more quickly 



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10-80 



