Runoff and sediment loads in the Tijuana River: Dam effects, extreme events, and change during urbanization

Study region: Tijuana River watershed on the US-Mexico border, where urbanization, estuarine sedimentation, and beach erosion incentivize quantification of runoff and suspended sediment loads (SSL) and concentrations (SSC). Study focus: Rainfall, runoff and SSL were quantified for 2001 & ndash;2019 (storm-wise) and 1962-2019 (annual) using sediment rating curves and bootstrapping to quantify uncertainty. New hydrological insights: Annual runoff increased for a given rainfall depth following channeli-zation and the start of imported water in 1978-79, and during urbanization over 1980-2019. SSC for a given runoff fell between 1970 s and 2000 s, but annual SSL increased severalfold due to higher annual runoff. Half the SSL over 2001-2019 occurred during nine storms. Nearly half (48 %) of annual SSL occurred in the six wettest years over 1962-2019, though years with recurrence interval 2-10 years accounted for 50 %. Neglecting the impact of dams on the SSL-rainfall relationship during extreme wet years over-estimated annual SSL by a factor of 7 and by up to 30x for the wettest year. Long-term mean suspended sediment yield (119 tons km(-2) yr (-1)) is similar to other southern California watersheds but much lower than observed (5000 tons km(-2) yr (-1)) in a small watershed that also drains to the Tijuana estuary. Accumulation of sediment in the estuary may be driven by undammed side canyons, and by low runoff that cannot transport sediment to the coast.

Automated summary

This study quantified rainfall, runoff, and suspended sediment loads in the Tijuana River watershed on the US-Mexico border. The results showed that urbanization and channelization led to an increase in annual runoff and suspended sediment loads. Neglecting the impact of dams on extreme wet years resulted in overestimation of suspended sediment loads.