Rivers and streams may look permanent, but their lengths can change dramatically with the seasons, according to a new study. It reports that stream networks in the United States expand up to five times their low-flow size during wet conditions. The findings offer the first large-scale insights into how water dynamically moves through landscapes and provide a framework for forecasting climate-driven changes in stream networks, particularly in response to increasing storminess. Traditionally regarded as static landscape features, stream networks exhibit significant seasonal and storm-driven fluctuations in their total wetted length. For example, seasonal rainfall and snowmelt saturate the landscape, temporarily increasing streamflow and extending stream networks. Additionally, changes in surface topography and permeability can cause networks to expand or contract. This responsiveness of stream length to shifts in landscape wetness is termed "network elasticity." Alongside variations in wetness, it governs fluctuations in stream extent across drainage basins that influence sediment transport, nutrient cycling, gas exchange, and aquatic habitats. However, understanding stream network length variability has generally been limited to small-scale studies relying on field measurements – and to a handful of small drainage basins. Here, Jeff Prancevic and colleagues used a semimechanistic model to estimate stream network elasticity for 14,765 basins across the contiguous U.S. Prancevic et al. found that the median stream network is five times longer during annual high-flow conditions than during low-flow conditions, with regional differences influenced by hydroclimate and topographic sensitivity. Mountainous and humid regions tend to have more stable stream networks. The findings suggest that a basin’s topography and subsurface structure, which determine its network elasticity, are as crucial as climate factors in predicting fluctuations in stream length. Although the study focuses on the continental U.S., the authors note that the methodology, which only requires digital elevation models and streamflow data, could be applied more broadly.

 

Raw data files for this paper are available to news outlets interested in building their own data visualizations. Please contact Jeff Prancevic at jeffp@ucsb.edu for more information.

Despite record rainfall in the region in early 2023, only a fraction of Southern California’s groundwater reserves has been replenished, researchers report. Their study, which leverages seismic noise data from across Greater Los Angeles, highlights the urgent need for improved monitoring and management of the state’s critical groundwater reserves. After enduring two decades of severe drought, California experienced an abrupt meteorological shift in water in 2023. A succession of 16 atmospheric rivers from late 2022 through early 2023, followed by the torrential rains of Hurricane Hilary in August, resulted in one of the wettest years on record. In the Greater Los Angeles region, rainfall reached 300% of historical norms. Although this period of intense precipitation rapidly replenished California’s surface reservoirs to 128% of their historical average, the extent of groundwater recovery has remained uncertain. Accurately quantifying changes in groundwater storage is essential for assessing drought impacts and ensuring sustainable water management. Yet, it remains a complex challenge, particularly at the basin-to-watershed scale essential for resource planning. Focusing on the Los Angeles area, Shujuan Mao and colleagues demonstrate that seismic noise analysis can effectively track groundwater storage changes with high resolution. Using ambient-field interferometry and passive data from 2003 to 2023 from a network of existing seismometers, Mao et al. measured variations in seismic wave speed; these variations reflect shifts in subsurface water distribution due to groundwater recharge. The regional seismic hydrograph revealed that the 2023 storms replenished only about 25% of the groundwater lost over the past two decades, highlighting substantial long-term depletion and a slow recovery of deep aquifers on a decadal scale. “Integrating the seismic imaging techniques of Mao et al. with existing methods, such as in situ well monitoring and satellite-based or aerial remote-sensing observations, would allow for comprehensive tracking of groundwater,” write Taka’aki Taira and Roland Bürgmann in a related Perspective. “This will help to ensure the preservation of sustainable water supplies for future generations.”