Hydrological response to an environmental flood: Pulse flow 2014 on the Colorado River Delta

Increasing pressure on water availability in the Colorado River Basin due to a long and severe drought, water over-allocation, increasing water demands, and a warming climate point toward the need to optimize use of water to meet all goals, including environmental restoration. In this paper, we analyze the hydrologic response of the Colorado River Delta to the 2014 pulse flow. In so doing, we identify hydrological criteria for optimizing the use of water for riparian restoration. We analyzed continuous hydrographs obtained from discharge measurement sites along the river channel, quantified areas inundated by water, and interpreted groundwater dynamics and their implications for riparian vegetation. Our most important finding is that 91.4% of the delivered water infiltrated into the first 61.2 km of the riverbed (between Morelos Dam and Pescaderos), recharging the underlying aquifer. This large volume of infiltration occurred mainly because several obstructions along the main channel impeded downstream surface flow, abandoned river meanders acted as infiltration basins, sandy riverbed and terrace sediments allowed for rapid infiltration, and a depressed groundwater table created a large unsaturated zone to fill. Most of the water was delivered at Morelos Dam. However, smaller water deliveries via Mexicali Valley's irrigation canal system bypassed the reaches of maximum infiltration, enabling the achievement of longitudinal river connectivity from Morelos Dam to the Gulf of California, and inundating important flood-dependent restoration sites. To optimize future environmental water deliveries, we encourage the use of irrigation infrastructure to deliver water directly to specific restoration sites to the extent possible, thereby avoiding reaches with high infiltration capacity and low riparian restoration potential. To improve river channel functionality in high-infiltration reaches, we recommend strategies to flood only the main channel and avoid off-channel depressions. By considering hydrological responses to environmental flow deliveries, riparian restoration goals can be achieved efficiently, even in highly controlled rivers with limited water availability.