Global River Volumes Fluctuate Less Than Previously Estimated, SWOT Mission Reveals

New data from the Surface Water and Ocean Topography (SWOT) mission indicates that global river volumes fluctuate less than previously estimated. The mission, led by NASA and the French space agency CNES, has tracked the swelling and shrinking of Earth's rivers over a year, also revealing new details about underwater river channel topography.

Key Findings: Less Fluctuation, New Insights

The SWOT analysis, covering nearly 1.6 million observations between October 2023 and September 2024, found that aggregate river volumes varied by approximately 83 trillion gallons (313 cubic kilometers).

This represents about 28% less fluctuation than the lowest prior estimates, a result potentially influenced by a record drought in the Amazon during the study period.

Specific observations include:

• The Amazon River experienced the largest variation, gaining and losing over 45 trillion gallons (172 cubic kilometers) over the study period.
• The Nile River showed less variation than anticipated, with changes of 2.2 trillion gallons (8.5 cubic kilometers). This could be due to factors such as upstream damming, drought conditions, or instrument calibration.

The SWOT Mission: A New Approach to Hydrology

Launched in 2022, SWOT is a collaborative effort by NASA, CNES, the Canadian Space Agency (CSA), and the UK Space Agency. It is the first satellite designed to survey oceans, lakes, and nearly all global rivers with high precision.

Unlike traditional methods that rely on models or separate altimeter and optical/radar estimates, SWOT measures both river width and surface height simultaneously. It uses its Ka-band Radar Interferometer (KaRIn) instrument, which bounces microwaves off the water's surface to time the signal's return. This method provides new insights into river storage changes and allows for the visualization of underlying riverbank and bed shapes, which have largely remained unmapped globally.

Broader Implications for Global Water Understanding

Scientists state that the findings contribute to answering fundamental questions about river flow rates and the input of rainwater and snowmelt. The data also offers a window into diverse river channel contours, ranging from concave to convex and steep to gentle.

Understanding these contours is crucial for navigation and flood prediction.

Researchers anticipate these insights will advance the understanding of how water moves through the global Earth system.