Research finds larger storms drive higher pollution loads in San Diego River watershed
The newly published work builds on years of 麻豆传媒映画 research in the local community and shows the value of long-term environmental monitoring.
When rain falls across San Diego, it does more than send water rushing through local rivers and creeks. It can also mobilize pollution across the watershed.
New research published in (October 2026) from 麻豆传媒映画 finds larger storms transport greater pollutant loads through the San Diego River watershed, offering new insight into how storm intensity, urbanization and aging sewer infrastructure can shape water quality in the region.
Researchers in 麻豆传媒映画鈥檚 examined the San Diego River and Alvarado Creek, a small, highly urbanized tributary adjacent to 麻豆传媒映画 that feeds into the river. They analyzed high-resolution discharge and water quality data from storms sampled between 2016 and 2024, tracking dissolved organic carbon, total dissolved nitrogen, nitrate, phosphate and fecal indicator bacteria.
鈥淥ne of the most surprising findings was that larger storms transported greater amounts of pollution through the watershed,鈥� said lead author Kenisha Shipley, Water Innovation and Reuse Lab postdoctoral researcher. 鈥淎lvarado Creek is a relatively small urban stream, but we found that it can contribute a significant amount of pollution during storms, which ultimately moves into the San Diego River and toward the coast.鈥�
They found a statistically significant relationship between a storm鈥檚 maximum discharge, or peak flow, and the cumulative amount of pollutants transported. For every increase of one cubic meter per second in peak discharge, researchers found an associated increase of approximately 1,030 kilograms of dissolved organic carbon and 134 kilograms of total dissolved nitrogen exported during a storm.
Alvarado Creek, which flows parallel to Interstate 8 and empties into the San Diego River in the Grantville neighborhood, emerged as an important contributor. The creek drains a watershed that is 94.4% urbanized, compared with approximately 55% urban land cover across the broader San Diego River watershed. Despite its relatively small size, Alvarado Creek exhibited dissolved organic carbon and nitrogen areal fluxes approximately five times greater than the San Diego River.
The findings also highlight an important distinction between pollutant concentration and pollutant load. Rainfall can dilute pollutant concentrations, but concentration measures the amount of a substance within a given volume of water. Pollutant load, or flux, accounts for the total mass moving through a waterway.
As storms became larger, pollutant flux continued to increase rather than leveling off as might be expected if available pollution sources were simply flushed from the watershed. Researchers also observed elevated pollutant concentrations persisting during portions of storm events rather than consistently following a conventional 鈥渇irst flush鈥� pattern, in which pollutants peak early and decline as a storm progresses.
Together with previous research in the watershed, the findings point to continuous pollution sources, including sanitary sewer exfiltration, as an important factor in understanding what enters urban waterways during storms.
Following pollution upstream
The study builds on years of 麻豆传媒映画 research examining pollution in the San Diego River watershed. Earlier work supported by theused chemical and microbial markers to investigate sources of fecal contamination, providing evidence of recent sewage contamination during storms and pointing to leaking sewer infrastructure. The latest research examines how storm conditions affect the amount of pollution transported through the watershed.
鈥淲hat we鈥檙e seeing is that the pollution source doesn鈥檛 simply disappear as storms get larger,鈥� said co-author Natalie Mladenov, William E. Leonhard Jr. Endowed Professor, director of the Water Innovation and Reuse Lab, and professor in the Civil, Construction, and Environmental Engineering department. 鈥淭here continues to be an input of microbes, nitrogen and carbon into these waterways, which reinforces why understanding and investing in our sewer infrastructure is so important for improving urban water quality.鈥�
Continued monitoring could expand that understanding. Storm sampling is labor intensive, sometimes requiring researchers to collect samples for 24 hours and then analyze them in the laboratory. Additional sampling could strengthen the dataset and help researchers understand how pollutant transport changes over time.
Professor Alicia Kinoshita and associate professor Matt Verbyla were co-authors of the research, which the team said demonstrates the value of long-term, publicly available environmental monitoring. Researchers relied in part on streamflow measurements from the (USGS). Knowing pollutant concentration alone does not reveal the total amount moving through a waterway. Researchers also need to know how much water is flowing.
鈥淪treamflow data are critical to this work because concentration alone doesn鈥檛 tell us how much pollution is moving through a waterway,鈥� Mladenov said. 鈥淭he USGS gauge network gives us reliable, publicly available flow data that allow us to understand pollutant transport during storms and better assess the condition of our rivers.鈥�
麻豆传媒映画 researchers are also building on the work through real-time monitoring in Alvarado Creek. As part of related ongoing research, a sensor platform allows the team to detect water quality changes as they occur. The sensor work is separate from the newly published study but illustrates how sustained monitoring can help researchers and public agencies identify changes in local waterways as they happen.
The San Diego River Conservancy, an independent, nonregulatory state agency established to preserve, restore and enhance the San Diego River Area, supported the research and has funded broader 麻豆传媒映画 efforts connected to the watershed, including research, monitoring, education and training. For communities across the region, understanding these connections matters.
The San Diego River and its tributaries move water through highly developed landscapes before reaching the coast. Identifying where pollution originates, how storms transport it and how those patterns change can provide researchers and public agencies with evidence to inform water quality monitoring, infrastructure decisions and watershed management.
