Water in the environment is often measured in acre-feet. An acre-foot is the volume of water it takes to cover an acre of land to a depth of one foot. (In gallons, that's 325,851.) Typically, 1 acre-foot of water would be enough to supply one to four suburban American households for a year.
As a benchmark for sizing up river flows or reservoir capacities, consider that the average runoff in the Sacramento River basin is 22 million acre-feet per year.