SPRINGS AND THE ROLE OF STRATIGRAPHY AND STRUCTURE ON STREAMFLOW GENERATION IN EL RITO WATERSHED

Recent research indicates that groundwater/surface water interactions are very complex in the El Rito watershed. In fact, this watershed appears to be a unique variation of the 3D watershed model [Frisbee et al., 2011] where interbasin groundwater flow (IGF) is thought to occur. The El Rito watershed can be broken into four distinct hydrogeological zones: 1) perennial streamflow in the headwaters maintained by springs and groundwater discharge, 2) losing conditions downstream of the headwaters, 3) a small, persistent 500 m gaining stretch in the mid-reach, and 4) losing conditions from the mid-reaches to the outlet. In this poster, we investigate the processes controlling spring generation in zone 1 and the gaining-to-losing transition in zone 2. The majority of perennial springs are located in the headwaters where east-dipping Tertiary conglomerates are underlain by west-dipping Mesozoic and older sedimentary layers. We hypothesize that the contact between the Tertiary and Mesozoic layers exerts primary control on the generation of springs in the headwaters and that this stratigraphy is responsible for the gaining/losing transitions in zone 2 as well. We tested this hypothesis by mapping geologic units near springs, aspect of spring emergence, age-dating of springs, and electrical resistivity surveys along spring emergences. Our data show that springs are more numerous in the northern extent of the headwaters and emerge on the western side of the stream valley primarily from the Tertiary conglomerates. In the southern extent of the headwaters, springs are less numerous and emerge from the eastern side of the stream valley where the Tertiary conglomerates have been completely removed and the stream has downcut into the west-dipping Mesozoic sedimentary layers. This is also the location where the stream transitions from gaining to losing. We infer that the Tertiary conglomerates are critical sources of groundwater for spring and streamflow generation in the headwaters. Groundwater in the Mesozoic sediments likely flows under El Rito creek and into the adjacent watershed to the west. Thus, groundwater/surface water interactions are strongest where the conglomerates are present. This research highlights the need to quantify the role of stratigraphy and structure on streamflow generation in large watersheds.