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24 The roughness of the streambed and the friction th,ereby caused is responsible for creating a characteristic boundary layer that is 1-3 1IlllI thick which may increase or decrease dependillg upon vel,)city and location. It is this "dead water zone" that provides e>,cellent habitat for benthic organisms. Stream Water TempeI'ature Water confers to stream environments a greater stability than is found in terrestrial and aerial situations (habitat) because "f its high I>pecific heat. The term specific heat describes that amount of heat in calories that is required to raise a unit weight of water 1 C. ~Iost other substances of the stream environment such as bedrock have much lower specific heats. In comparison to non-aquatic environmen,ts, stream temperature fluctuation tends to occur very gradually, s'easonally and diurnal extremes are small. The surface water temperature within the <emperate zone (e.g., North America) is determined mainly by air temperat,":"(! during each season of the year. Marked variations occur in air temperatures at high and low elevations (altitude), from north to south (latitude) and affect the patterns of stream temperature. The amplitude of diel (daily) fluctuations and the magoitude and rate of seasonal temperature change vary with geographic locat.lon and stream size. The highest summertime surface water temperatures are in parts of the southwest (Southern California, Arizona, Texas). The coldest temperatures are in the Rocky Mount~ins of Colorado and Wyoming and along the west coast. Marked variations in temperature patterns in the western Rocky Mountains and the Appalachian Mountains refle'~t the rugg,ed topography. Surface water temperatures show a gradual decrease proce'eding from south to north in the' central states where thEl topograph:, is relatively flat. Temperature variation vsries both within and between the twenty..one principal U.S. drainage basins and reflects differences Jln climate, regional ground water temperatures, channel geometry, riparian vegetlltion and the watershed characteristics. Stream discharge durJ,ng periods of no surface runoff is maintained largely by ground. water accI~ual along headwater tributaries. Therefore headwater stream temperatures tend to be dolllinated by ground water outflow. Ground water generally ranges within 1 C of lIean annual air temperature of a drainage basin, vades little seasonally, anej. shows a well defined latitudinal gradient ranging fre,m about 3 C in Maine to about 25 C in southern Florida (l18:ure 15). Although the temperature of ground water may vary from freezing in caves and t~neath glaciers and snow fields to above the boiling point in super heated geysers, the normal temperature at shallow depths ranges from a low of about 3-1 C (37 F) to a high of slightly above 25.2 C (77 F). Normal' ground water temperatures at a depth of 30 to 60 feet seldom vary mOl'e than a degree or so all year long. Lowest ground water tempel'atures are found in northern Minnesota and the highest temperatures are felund in the E,xtreme southern tip of Florida. Like surface water temperaturu, ground war,er temperature. shOw a IIOre or less regular increllse from no,rt!: to south.