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.
1 So a gallon of salt water weighs 8.556 lbs.  That's a common expression. A pint is 16 fluid ounces and 16 fluid ounces of water weighs approximately 16 dry ounces. Two pints to the quart, four quarts to the gallon means eight pints to the gallon, so a gallon weighs about eight pounds. But that only applies to FRESH water. Salt water has a whole range, depending on how much salt is dissolved in the water. So you'd have to add the weight of the dissolved salt.