Print PageTuesday, August 7, 2012: 10:50 AM
C123, Oregon Convention Center
Background/Question/Methods:
Regulated flows on western rivers are known to change flood regimes, reduce flow variability, and impact riparian vegetation in a number of ways. On the dam-controlled Dolores River, the timing and frequency of peak flows due to snowmelt are relatively unchanged, but the magnitude of such flows is significantly lower. We have documented declines in the cover of cottonwood woodlands below the dam, and hypothesize that these changes are due to a lack of groundwater recharge. Here we report the results of three years (2010-2012) of shallow groundwater well data and two years of on-site precipitation and soil moisture data from three sites on the Dolores River: one above and two below McPhee Dam. Each site contains a transect of three piezometers with pressure transducers to measure water height and barometric pressure, as well as a tipping rain gauge to measure site specific precipitation events. Quantification of river discharge and dam releases are compiled from US Geological Survey and Colorado Division of Water Resources stream gauges near the study sites.
Results/Conclusions:
Wells located in the willow, cottonwood, and more drought-tolerant forb/shrub vegetation zones indicate that while the Dolores below the dam has substantially reduced flows, the floodplain does still experience some recharge during seasonal flow events, with the most significant recharge occuring during longer duration high flows in spring. However, a twenty-year data set from one Dolores River site shows changes in vegetation composition post-dam; new evidence from groundwater wells indicates a possible link with recharge - or lack thereof. While shrubby phreatophytes on the riverbank have increased in cover (due to lack of scouring spring floods), canopy cover of the dominant cottonwoods has declined, especially in the wake of severe drought. Using our well data, we extrapolated that the low river flows during the 2000-2003 drought cycle led to a lack of groundwater recharge in the floodplain and a loss of the connection between cottonwood root systems and the water table. This is consistent with on-site rain gauge data from 2011 which demonstrate that higher elevation sites are recharged by both precipitation and hyporheic flow associated with high stream flows, but at the more drought sensitive lower elevations, sites are dependent on hyporheic flow and bank storage to recharge the aquifer. At these sites, cottonwood cover has declined significantly, especially during & after prolonged drought.
Regulated flows on western rivers are known to change flood regimes, reduce flow variability, and impact riparian vegetation in a number of ways. On the dam-controlled Dolores River, the timing and frequency of peak flows due to snowmelt are relatively unchanged, but the magnitude of such flows is significantly lower. We have documented declines in the cover of cottonwood woodlands below the dam, and hypothesize that these changes are due to a lack of groundwater recharge. Here we report the results of three years (2010-2012) of shallow groundwater well data and two years of on-site precipitation and soil moisture data from three sites on the Dolores River: one above and two below McPhee Dam. Each site contains a transect of three piezometers with pressure transducers to measure water height and barometric pressure, as well as a tipping rain gauge to measure site specific precipitation events. Quantification of river discharge and dam releases are compiled from US Geological Survey and Colorado Division of Water Resources stream gauges near the study sites.
Results/Conclusions:
Wells located in the willow, cottonwood, and more drought-tolerant forb/shrub vegetation zones indicate that while the Dolores below the dam has substantially reduced flows, the floodplain does still experience some recharge during seasonal flow events, with the most significant recharge occuring during longer duration high flows in spring. However, a twenty-year data set from one Dolores River site shows changes in vegetation composition post-dam; new evidence from groundwater wells indicates a possible link with recharge - or lack thereof. While shrubby phreatophytes on the riverbank have increased in cover (due to lack of scouring spring floods), canopy cover of the dominant cottonwoods has declined, especially in the wake of severe drought. Using our well data, we extrapolated that the low river flows during the 2000-2003 drought cycle led to a lack of groundwater recharge in the floodplain and a loss of the connection between cottonwood root systems and the water table. This is consistent with on-site rain gauge data from 2011 which demonstrate that higher elevation sites are recharged by both precipitation and hyporheic flow associated with high stream flows, but at the more drought sensitive lower elevations, sites are dependent on hyporheic flow and bank storage to recharge the aquifer. At these sites, cottonwood cover has declined significantly, especially during & after prolonged drought.
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