An upside-down river: Impoundments and eutrophication alter downstream predictions of water quality in the Klamath River, Oregon

Background/Question/Methods

Large river impoundments are ubiquitous features in many parts of the world. The River Continuum Concept (RCC) and the Serial Discontinuity Concept (SDC) are two examples of widely acknowledged models of river ecosystems that suggest longitudinal shifts in parameters in response to perturbations such as impoundments.  While these concepts may have broad utility, they inadequately address how nutrient enrichment (eutrophication) may alter predictions within regulated rivers.  The objectives of our study were to investigate these predictions by determining longitudinal patterns of water quality parameters and organic matter composition within the Klamath River, Oregon/California.  We collected monthly water samples for one year at nine sites over 130 miles on the Klamath River, beginning at the headwaters and sampling above and below six reservoirs.  Samples were analyzed for dissolved oxygen, conductivity, pH, temperature, turbidity, suspended solids, nutrients (TN, TON, NH₄⁺, NO₃^-, TP, TDP, SRP, DOC), and organic matter (OM) composition (chlorophyll a, UV-absorbance, fluorescence index, biological oxygen demand (BOD)). 

Results/Conclusions

Our results indicate that the Klamath River functions as an “upside down” river in terms of many of the predictions based on river ecosystem concepts such as the RCC and SDC.  Conditions in the headwaters were the most degraded and generally improved below dams and with increasing distance downstream.  The highest concentrations (TN= 3.249 mg L^-1, NH₄⁺= 0.124 mg L^-1 , TP= 0.223 mg L^-1, DOC= 9.67 mg L^-1), the most labile OM, and the highest BOD (20-day BOD = 42.7 mg L^-1) were observed in the headwaters during the summer months.  SRP was more variable, but generally remained similar throughout the river or increased slightly in the downstream direction, likely as a result of lower N:P ratios and reduced SRP uptake.  Using a general linear mixed model, we determined a significant effect of position above or below a dam, depending upon time of year, on TN, TON, suspended solids, BOD, and OM composition.  Overall, downstream improvement in water quality likely results from storage and processing of OM in reservoirs and dilution effects from groundwater and tributaries.  Four out of six dams on the Klamath River are planned for removal in the next decade and these results suggest that the removal of downstream reservoirs may affect the transport of nutrients and organic matter, potentially increasing downstream impairment in the summer months. Dam removal should therefore be considered in conjunction with the restoration of upstream conditions.