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.
Each gallon of sea water contains about 4.5 ounces of salt, which is about 3.5 percent of its total weight. Almost all of the other answers here were correct (with varying degrees of precision) but essentially said that water weighs (for a US gallon) 8.34 pounds so multiply by 5 and your answer is lbs.