Hydrologic Controls On Removal of Oxygen In The Bed of A Mountain Stream, East River, Colorado

Cantrell, Erin M.

Graduate Thesis Or Dissertation

Hydrologic Controls On Removal of Oxygen In The Bed of A Mountain Stream, East River, Colorado Público Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/3x816n87r

Abstract

Dissolved oxygen (DO) concentrations in rivers are critical for aquatic habitat and controlled by biological generation and uptake, and physical factors. One important physical factor is hydrology: not only streamflow dynamics (changing amounts of water), but also changes in surface-groundwater exchanges. Over a period of 15 months in East River, Colorado from August 2017 (a somewhat ‘average’ flow year) to October 2018 (a low flow year) high frequency (5 minute) DO and temperature data were collected in the water column of the river and directly in the streambed at depths of 10 cm, 20 cm, and 35 cm. Using the VFLUX2 model, temperature data were used to estimate vertical upwelling and downwelling vertical fluxes of water (and solutes). We find that there was downwelling throughout both years, and increased fluxes into the bed during peak flows. From relating vertical flux to steam discharge and groundwater tables we find that stream discharge is a control of streambed DO during low flow. We calculated DO removal from the channel into the bed, finding enhanced removal rates in 2018 compared to 2017. We observed an extended hyporheic anoxic period throughout the summer and fall of 2018 due to the increased DO removal rates within the hyporheic zone. The three subsurface locations were found to not all be on the same flow path, which may account for some non-linear DO concentration decreases with depth in 2017. Anoxia (zero DO concentration) is observed briefly in 2017 and for an extended period in 2018. Increased DO removal due to low flow conditions and increased temperature are the primary factors for anoxia in 2018. The anoxia patterns differ between the two years, so it is likely that the cause of anoxia differs. To better understand the system dynamics, three periods of atypical DO or vertical flux were analyzed as more detailed small events. These helped us to understand system storm responses and causes of anoxia. This research has advanced our understanding of the dependence of DO in both the streambed and open channel on streamgroundwater exchanges by showing periods of stream discharge control on DO dynamics and increased DO removal at low flow and high temperature.

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