Fish, being cold-blooded, are hostage to their surrounding temperatures. At winter water temps, trout are zombies, in a virtual coma. Rainbows and browns are happiest in the low 60's, brook trout about 5 degrees lower. Water in the 70's feels to them like air in the 90's does to us. If hooked then, they may die from the stress.
A thermometer can lead you to a honey-hole. If you can find a place where a tributary or obscure spring outflow is several degrees cooler than the main stream, trout will be there in hot weather.