Effects of river regulation on water quality in the lower Mokelumne River, California

Abstract

This study examines the effects of flow regulation on water quantity and quality by comparing an impounded system (Mokelumne River) with an adjacent unimpounded system (Cosumnes River). Between 1999 and 2002, the Cosumnes River displayed a strong seasonal cycle for each constituent analysed (total suspended solids, NO 3 -N, total nitrogen, PO 4 -P, total phosphorus, dissolved silicon, specific conductivity, flow), while reservoirs in the lower Mokelumne buffered and attenuated physical and chemical fluctuations creating a weak seasonal pattern. Dissolved silicon and total suspended solids were the two constituents most efficiently sequestered by the reservoirs. While the reservoirs acted as traps for most constituents, NO 3 -N and PO 4 -P were produced during the drier years of the study, 2001 and 2002. In contrast, the unimpounded reference reach in the Cosumnes was an annual source for all constituents measured. The Cosumnes delivers its highest NO 3 -N concentrations during the winter months (December-April), while peak concentrations in the Mokelumne occur during the snowmelt (May-July) and baseflow (August-November) seasons. Due to downstream N-limitation, this temporal shift in NO 3 -N export may be contributing to accelerated algal growth in the reach immediately downstream and eventually to algal biomass loading to the downstream Sacramento-San Joaquin Delta.

The upside-down river: Reservoirs, algal blooms, and tributaries affect temporal and spatial patterns in nitrogen and phosphorus in the Klamath River, USA

“…This is particularly important in systems such as the Klamath River, where dam removal and anadromous fish restoration are being considered but basic water quality variables, such as temperature, may have limited capacity for improvement and so addressing nutrients may be a more viable restoration goal. Dam removal can alter the downstream fate and transport of nutrients and water quality, and can shift the role of a river reach as a nutrient source or sink (Stanley and Doyle, 2003;Ahearn et al, 2005;Doyle et al, 2005). While specific recommendations regarding dam removal are beyond the scope of this paper, the nutrient dynamics described herein should be carefully considered when weighing options for dam removal and fish restoration on the Klamath River.…”

“…On one occasion (October 2010), increased NH 4 + was observed below Iron Gate, possibly from mineralization of TON emanating from upstream reservoirs at the end of the postbloom period. In addition, occasional increases in NO 3 À were observed during the winter low and winter/spring high periods below both Keno and Iron Gate Reservoirs, and may reflect discordance between the accumulation of nutrients and flushing from the reservoir (Ahearn et al, 2005). Higher winter concentrations of NO 3 À may reflect longer residence times and the accumulation of TON, followed by mineralization and nitrification prior to export.…”

“…Impoundments often create reservoirs that disrupt lotic connectivity and alter in-stream biogeochemical cycling (Friedl and Wüest, 2002;Humborg et al, 1997), organic matter dynamics (Miller, 2012), and downstream transport of sediments and nutrients (Kelly, 2001;Houser et al, 2009). Timing of the processing and export of materials may also be disrupted, leading to seasonal changes in downstream productivity (Ahearn et al, 2005). For example, lentic bodies are composed of still waters and are frequently viewed as annual net sinks for phosphorus and nitrogen (Harrison et al, 2009), but are also important sub-annually by influencing the export of nutrients to downstream reaches (Wurtsbaugh et al, 2005;Kendall et al, 2001).…”

Sediment and nutrient dynamics following a low-head dam removal at Murphy Creek, California

“…Before removal, the majority of nutrients in an impounded reach are stored in the benthos (Perrin et al 2000;Stanley and Doyle 2003;Ahearn et al 2005). With drawdown, the reservoir sediment is exposed and complex biogeochemical reactions begin.…”

Past, Present, and Future Nutrient Quality of a Small Southeastern River: A Pre-Dam Assessment

“…Reservoirs not only increase certain chemical concentrations to harmful levels, but also reduce needed parameters to critical levels by acting as annual sinks for organic and inorganic compounds [7]. Even though a reservoir may act as a flow-through system, dams often act as large nutrient traps that slow nutrient advancement [3].…”