River Streamflow: Tracking How Much Water the West Has

River level tells you how high the water is; river streamflow tells you how much water is actually moving — the volume coursing past a point each second. It's the truest measure of a river's health and a region's water supply. This guide tracks streamflow on three major U.S. rivers against their long-term averages and shows what it reveals about drought, especially on the strained Colorado.

What is streamflow?

Streamflow (or discharge) is the volume of water flowing past a gauge, usually in cubic feet per second. Here it's shown as a percentage of each river's long-term average, so 100% is normal, above means a wetter-than-usual stretch, and below means drier. Plotting it as a smoothed percentage lets you compare very different rivers — a small Eastern river and a giant Western one — on the same scale, revealing whether each is running rich or short.

The Colorado River's persistent shortfall

The standout story is the Colorado River, which has spent long stretches running below its average — a direct reflection of the multi-decade megadrought gripping the Southwest. Because the Colorado supplies water to some 40 million people and vast farmland across seven states, every percentage point below average translates into less water reaching reservoirs like Lake Mead and Lake Powell, tighter allocations, and harder choices downstream. Persistent below-average flow is the upstream cause of the reservoir crisis.

Wetter and drier rivers

The other rivers tell their own stories. Flow swings with rainfall and snowpack — a big snow year in the Rockies or a wet season in the East sends a river well above average, while drought drops it below. Eastern rivers like the Potomac tend to be flashier, responding quickly to rain, while big snowmelt-fed Western rivers like the Columbia follow the slow rhythm of the mountain snowpack. The lines crossing above and below the 100% baseline trace each region's cycle of wet and dry years.

Why streamflow matters

Streamflow is one of the most important numbers in water management. It determines how much water is available for cities, farms, and hydropower; whether rivers are navigable for barges; the health of fish and ecosystems; and the risk of both drought and flooding. Water managers watch it constantly to decide reservoir releases and allocations. In a warming climate that's shifting where and when precipitation falls, tracking streamflow is essential to anticipating the next water crisis before the reservoirs run low.