Hydrological Controls on Nutrient Concentrations and Fluxes in California Park Lakes, Butte County, California

Storm drains, groundwater wells, and an ephemeral stream network, Dead Horse Slough, supply nutrients to California Park Lakes, three interconnected reservoirs east of Chico, California. The 11 km² watershed above the lakes traverses the Tuscan Formation, which consists of volcanic/sedimentary rocks with thin soil and suburban housing developments. The relatively impermeable Tuscan Formation strata and developed surfaces are consistent with surface flow strongly exceeding subsurface flow. During the winter months, storms supply both forks of the Sloughs, outflow occurs by flow over the terminal dam, and lake water is repeatedly flushed. During summer months little to no water flows in the slough, so water levels in the lakes are maintained by pumping groundwater into the system from the underlying aquifer and from storm drain runoff, which is derived from about 20 suburban microwatersheds surrounding the lakes. Water mass balance analyses (Murphy and Sundermann, 2002) indicated that evaporative losses from the lakes during summer are comparable to water supply from wells and surface runoff. Drainage areas and land use patterns have been mapped to examine relations between flow and nutrient supply. Data have been collected for over two years on runoff volumes and nutrient concentrations from microwatersheds surrounding the lakes. The analyses indicate that most nutrients in inflow during the summer time are supplied from groundwater wells. Microwatershed land use has a greater effect on nutrient supply than the catchment area because of both higher runoff (mostly from lawn watering) and higher nutrient concentrations in developed microwatersheds.