UC Merced

The Merced River, originating in the Sierra Nevada in California, drains a watershed with an area of approximately 3,305 km2. The stream has been highly altered due to diversions, gold-dredged mining, damming, and subsequent modification to the hydrograph. Over the course of a year, groundwater-surface water interactions were studied to elucidate the hydrological connection between the Main Canal, an unlined engineered channel containing Merced River water and flowing parallel to the river (average elevation 89 m) and also a highly conductive previously-dredged floodplain, and the Merced River average (elevation 84 m). Upstream of the study reach, located in an undredged portion of the floodplain, are a state run salmon hatchery and a privately run trout farm that have been operating for approximately 40 years. Exchanges between the hyporheic and surrounding surface, groundwater, riparian, and alluvial floodplain habitats occur over a wide range of spatial and temporal scales. For this study, pressure transducers were installed in seven wells and four ponds located in the dredged floodplain. All wells were drilled to the Mehrten Formation, a confining layer, and screened for the last 3 m. These groundwater well water levels as well as the surface water elevations of the Main Canal and the Merced River were used to determine the direction of sublateral surface flows using Groundwater Vistas as a user interface for MODFLOW. The wells, the canal, the river, and seepage from the river banks were sampled for major anion and cation, dissolved organic carbon, total nitrogen, total iron, and total dissolved iron concentrations to determine water sources and the possibility of suboxic water. Field analysis indicated that water in all wells and ponds exhibits low dissolved oxygen, high conductivity, and oxidation/reduction potentials that switched from oxidizing to reductive during the course of the monitoring period. Chemical analysis indicates that there are three sources of water for this floodplain: the Merced River and Main Canal (which are chemically very similar), the waters from the trout farm, and precipitation. The well closest to the trout farm had a C:N of 1, typical of a highly carbon-limited system. MODFLOW particle tracer experiments designed with homogeneous soils were performed and did not capture the near-surface preferential flow paths. These results indicate that travel time between the Main Canal and Merced River is approximately 10-15 years, while chemistry results indicate seasonal fluctuations. Based on the well levels and chemistry, this water system responds on a much faster scale than indicated by the particle tracer experiments. Reconciling these results, there must be significant preferential flow paths. Candidate flow paths are abandoned channels from the dredging era. The hydraulic gradient set up by the groundwater connection between Main Canal and the Merced River ensures that any effluent released by the trout farm will be transported to the Merced River. Conclusions of the year-long study are that the waters that seep from the Main Canal to the Merced River in this area can be suboxic, which is not conducive to spawning and incubation for native Chinook salmon (Oncorhynchus tshawytscha) an indicator species for the overall ecosystem's health. This study reach has been historically important for salmon spawning and rearing, as the area examined is where more than 50% of the Chinook salmon of the Merced River spawn. Currently, salmon restoration efforts are focusing on gravel augmentation and adding a side channel, but ignoring groundwater influences. Due to the causal connections between the hydrological system of the Merced River floodplain and the riverine system, habitat rehabilitation should target not only the surface water but also important subsurface hydrological components.