Tie a three-way swivel to your main line. Attach a foot-long section of monofilament to the second eye and tie it to a waterproof thermometer (you can get one from llbean.com for $12; 800-441-5713).
Attach a 1-ounce sinker to the swivel's third eye using a shorter section of monofilament so that the sinker doesn't bang up the thermometer.
To take the lake's temperature, cast, allow your thermometer to sink, and let it settle long enough for the temperature to register (about 15 seconds). Reel in quickly and read it. Make note of that number.
To locate a spring, start prospecting in likely areas. In rivers, begin in pools, deep runs, or near feeder streams. In lakes and ponds, look near inflows or outflows, around rocky areas, or in transition zones where the bottom changes from sand to rock. If the water is clear enough, look for patches of overturned leaves or discolorations on a sandy bottom; these often indicate the presence of a spring.
When you locate a spot that's colder than normal, mark it on a map or GPS. --TOM KEER