Design floods serve an important role in environmental planning and management; however, flood frequency analyses often assume that historical records are stationary despite anthropogenic changes across the watershed. Such changes are especially prevalent in the southeastern United States where rapid population growth coupled with climate change is dramatically altering catchment response. To understand how design floods have changed over time, this study investigates historical records of

more »

... river discharge at over 5,800 USGS gauges in the Southeastern USA. When looking at trends in daily discharges, we find that 40% experienced a significant (p<0.05) increase in daily discharge, 50% experienced significant decrease, and 10% experienced no significant change in daily discharge. We observe that, in general, gages exhibiting increasing daily discharge are spatially concentrated east of the Appalachian Mountains and gages exhibiting decreasing daily discharge are spatially concentrated to the west. When looking at trends in extreme discharges using both the annual maxima and a peak-over-threshold with discharges above the 90th and 99th percentiles, we find that less than 20% of gages had a significant trend in the extreme discharges. We then conducted a flood frequency analysis at gages with the most significant (positive and negative) trends in daily discharge to estimate the 25- 50- and 100-year design flows using the Gumbel distribution based. We compare the modeled results using the annual maxima over the entire period of record and for every 30-year segment of the record. For all return periods, we find that about 75% of gages experienced a significant trend, with 50% of gages experiencing an increasing trend in the magnitude of the design flood. Future work will look to attribute our observed trends to changes in the watershed including urbanization and climate change. Understanding non-stationary trends in river discharge, especially in coastal zone, is critical for local government planning and adaptatio [...]