[1] We monitored summertime base flow water temperatures of hyporheic discharge to surface water in main, side, and spring channels located within the bank-full scour zone of the gravel-and cobble-bedded Umatilla River, Oregon, USA. Diel temperature cycles in hyporheic discharge were common, but spatially variable. Relative to the main channel's diel cycle, hyporheic discharge locations typically had similar daily mean temperatures, but smaller diel ranges (compressed by 2 to 6°C) and desynchronized phases (offset by 0 to 6 h). In spring channels (which received only hyporheic discharge), surface water diel cycles were also compressed (by 2 to 6°C) and desynchronized (by À4 to 6 h) relative to the main channel, creating diverse daytime and nighttime mosaics of surface water temperatures across main, side, and spring channels, despite only minor differences (<1°C) in daily mean temperatures among the channels. The river's hyporheic zone received and stored heat from the channel, yet hyporheic return flows carried heat back to the channel minutes to months after removal. Associated surface water temperature dynamics were therefore complex. Hyporheic discharge was not simply ''cooler'' or ''warmer'' than main channel water. Instead, instantaneous temperature differences between channel water and hyporheic discharge typically arose from diel temperature cycles in hyporheic discharge that were buffered and lagged relative to diel cycles in the main channel.

[1] We studied discharge data from stream gauges located in natural and anthropogenically modified river basins of the Northern Rocky Mountains over 59 years. We applied linear and nonlinear models to the data to determine what, if any, alterations have occurred in the annual flow regimes. By comparing the different results from the natural and anthropogenically modified river basins, we were able to distinguish the impacts that climate change and anthropogenic modifications have had on flow regimes in the Northern Rocky Mountains over the period of record. We found that regional climate change has not significantly altered the natural flow regimes. However, we did find an underlying cyclical pattern in the total amount of annual discharge in both the natural and anthropogenically modified river basins. This suggests a strong link between the quantity of runoff and some kind of hydroclimatologic cycle. Conversely, direct basin anthropogenic modifications such as damming, irrigation, and urbanization have caused a decrease in the difference between the minimum and maximum annual discharges and a decrease in the daily variations of flows across the Northern Rocky Mountains. In general, direct anthropogenic modifications of the river basins have altered the flow regimes to a much greater extent than climate change.

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Moore, Johnnie N., Alicia S. Arrigoni, and Andrew C. Wilcox, 2012. Impacts of Dams on Flow Regimes in Three Headwater Subbasins of the Columbia River Basin, United States. Journal of the American Water Resources Association (JAWRA) 48(5): 925‐938. DOI: 10.1111/j.1752‐1688.2012.00660.x Abstract:  We compared long‐term changes in flow regimes resulting from climate change with those resulting from dams in three matched pairs of natural and modified headwater subbasins of the Columbia River. Based on the analysis of 12 flow‐regime metrics, we found that damming had minimal effect on most quantity of flow metrics, but major effect on timing of flow metrics, especially those representing “spring runoff.” In all modified subbasins, “spring runoff” metrics occurred much earlier than natural flow (up to ∼44 days earlier for April‐July flows). Storage capacity modulated the magnitude of timing of flow‐metric changes, with the largest storage capacity leading to the most change. However, even in subbasins with low storage capacity, we found significant change in most timing of flow metrics. We also found that damming, especially in subbasins with higher storage capacity, overwhelmed climate variability in all basins for most flow metrics. This shows that reservoir operations need to be modified to more closely match the natural timing of flow regimes to promote positive ecologic response in modified rivers, even in basins where quantity of flow metrics have not changed substantially as a result of damming.