Of course, the main influence on streamflow is precipitation runoff in the watershed. Rainfall causes rivers to rise, and a river can even rise if it only rains very far up in the watershed-remember that water that falls in a watershed will eventually drain by the outflow point.
One US gallon of water weighs 133.526464 ounces. The calculations for that answer are below: 1 gallon = 8.345404 pounds 1 pound = 16 ounces 8.345404 X 16 = 133.526464.
Google says water weighs 8.344 pounds per gallon at freezing and 7.996 pounds per gallon at boiling with 8 1/3 pounds per gallon as a reasonable estimate for normal temperatures. So a 5 gallons of water will weigh about 42 pounds plus the weight of the container, depending on the temperature.