Variables that affect a waterway’s temperature include:

Variables that affect a waterway’s temperature include:

1. The color of the water. Most heat warming surface waters comes from the sun, so waterways with dark-colored water, or those with dark muddy bottoms, absorb heat best.
2. The depth of the water. Deep waters usually are colder than shallow waters simply because they require more time to warm up.
3. The amount of shade received from shoreline vegetation. Trees overhanging a lake shore or river bank shade the water from sunlight. Some narrow creeks and streams are almost completely covered with overhanging vegetation during certain times of the year. The shade prevents water temperatures from rising too fast on bright sunny days.
4. The latitude of the waterway. Lakes and rivers in cold climates are naturally colder than those in warm climates.
5. The time of year. The temperature of waterways varies with the seasons.
6. The temperature of the water supplying the waterways. Some lakes and rivers are fed by cold mountain streams or underground springs. Others are supplied by rain and/or surface run-off. The temperature of the water flowing into a lake, river or stream helps determine its temperature.
7. The volume of the water. the more water there is, the longer it takes to heat up or cool down.
8. The temperature of effluents dumped into the water. When people dump heated effluents into waterways, the effluents raise the temperature of the water.

Fish and most aquatic organisms are cold-blooded. Consequently, their metabolism increases as the water warms and decreases as it cools. Each species of aquatic organism has its own optimum (best) water temperature. If the water temperature shifts too far from the optimum, the organism suffers. Cold-blooded animals can’t survive temperatures below 0 ^oC (32 ^oF), and only rough fish like carp can tolerate temperatures much warmer than about 36 ^oC (97 ^oF).
Fish can regulate their environment somewhat by swimming into water where temperatures are close to their requirements. Fish usually are attracted to warm water during the fall, winter and spring and to cool water in the summer. Did you ever notice how fish swim down to the cooler parts of the lake to escape the heat of the noonday sun? Fish can sense very slight temperature differences. When temperatures exceed what they prefer by 1-3 ^oC, they move elsewhere!
Fish migration often is linked to water temperature. In early spring, rising water temperatures may cue fish to migrate to a new location or to begin their spawning runs. The autumn drop in temperature spurs baby marine fish and shrimp to move from their nursery grounds in the estuaries out into the ocean, or into rivers, as the case may be. As you can see, all sorts of physiological changes take place in aquatic organisms when water temperatures change.

Table 8. Water temperature and fish behavior
Temperatures are given as ºC (°F)
Fish Species	Optimum Temp	Above this temperature*:
		Fish will not spawn	Fish embryos die	Fish growth stops	Fish die
Atlantic Salmon	--	5 (41)	11 (52)	20 (68)	23 (75)
Black Crappie	--	17 (63)	20 (68)	27 (81)	--
Bluegill	--	25 (77)	34 (93)	32 (90)	35 (95)
Brook Trout	--	9 (48)	13 (55)	19 (66)	24 (75)
Carp	32 (90)	21 (70)	33 (91)	--	36 (97)
Channel Catfish	--	27 (81)	29 (84)	32 (90)	35 (95)
Coho Salmon	20 (68)	10 (50)	13 (55)	18 (64)	24 (75)
Emerald Shiner	--	24 (75)	28 (82)	30 (86)	--
Lake Herring (Cisco)	--	2 (36)	8 (46)	17 (63)	25 (77)
Large Mouth Bass	23.5 (74)	21 (70)	27 (81)	32 (90)	34 (93)
Northern Pike	--	11 (52)	19 (66)	28 (82)	30 (86)
Rainbow Trout	13 (55)	8 (46)	15 (59)	19 (66)	24 (75)
Sauger	--	12 (54)	18 (64)	25 (77)	--
Small Mouth Bass	--	17 (63)	23 (73)	29 (84)	--
Sockeye Salmon	15 (59)	10 (50)	13 (55)	18 (64)	22 (72)
White Sucker	--	10 (50)	20 (68)	28 (82)	--
Yellow Perch	--	12 (54)	20 (68)	29 (84)	32 (89)
-- indicates information not available. * The two left columns below this heading are a summary of reported values for maximum weekly average temperature for spawning and short-term maximum for embryo survival during the spawning season.  The two right columns are examples of calculated values for maximum weekly average temperatures for growth and short-term maximum for survival of fish during the summer. References 1,2

Fish are not the only organisms requiring specific temperatures. Diatoms seem to grow best at a temperature of 15-25 ^oC, green algae at 25-35 ^oC, and blue-green algae at 30-40 ^oC.
Warm water also makes some substances, such as cyanides, phenol, xylene and zinc, more toxic for aquatic animals. If high water temperatures are combined with low dissolved oxygen levels, the toxicity is increased.