Monitoring the Impacts of the Rim Fire on Tuolumne River Water Quality

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The Rim Fire has burned over 400 square miles of the Tuolumne River and Merced River watersheds in central California and is now the 3rd largest wildfire in state history. The burn area is largely on the Tuolumne between Hetch Hetchy Reservoir and Don Pedro Reservoir, both of which serve as critical sources of drinking water and irrigation water to San Francisco Bay area and Central Valley residents. Previous studies have shown that the impacts of forest fires on the hydrology and water quality of a basin can be significant and thus important to water managers; wildfires have the potential to alter soil infiltration and overland flow, resulting in changes in the timing and magnitude of annual and peak flows in a watershed, and they have been shown to increase the fluxes of sediment, nutrients, trace metals, polycyclic aromatic hydrocarbons (PAHs) and other water quality constituents of concern. The timescales of water quality impacts also vary, with the greatest changes in flows and sediment transport shortly after the event followed by longer-term (e.g. months to years) changes in biological pools and nutrient processes.

Location of the Rim Fire burn area and sensor measurements in Don Pedro Reservoir.

Location of the Rim Fire burn area and sensor measurements in Don Pedro Reservoir

Understanding the impacts of the Rim Fire on both the volume and quality of runoff water — particularly during the first winter storms and snowmelt following the fire - is therefore critical to maintaining safe drinking water and ecosystem health. The managers of Don Pedro Reservoir, Modesto Irrigation District (MID), Turlock Irrigation District (TID), City & County of San Francisco (CCSF), are particularly interested in real time data -- flow, sediment and water quality. This type of information can help them adjust their water treatment processes and determine appropriate actions to take in the plant, lake, and watershed.

The objective of the proposed work is twofold; (1) to monitor the water quality and sediment characteristics in the Tuolumne River during water year 2014 (~Oct. 2013 - Oct. 2014) (and beyond if funding is available) so that MID and TID have useful real-time data; and (2) establish a database of the first year of post fire flow and water quality information.

While characterizing the short-term water quality impacts is important, the time scale of water quantity and quality impacts by fire has been shown to be on the order of 3-10 years (Meixner, 2004), with a large variation of impacts between basins. It will take numerous years to fully quantify the impacts of the Rim Fire on flows and water quality, and this work will serve as the basis for a longer term monitoring effort. We have established a gage with water quality sondes on the Tuolumne River, currently at Wards Ferry, and we will deploy water quality instrumentation in the upper reaches of Don Pedro Reservoir (Figure 1) to evaluate the cumulative effects of the fire and potential water quality impacts on the reservoir.

We are using a combination of in situ measurements of flow and water quality and periodic discrete sampling to characterize water quality impacts from the Rim Fire. In order to establish a gage site before the storms this winter, a gage was established at Wards Ferry with separate funding from DWR. It includes a five parameter YSI sonde for comparison to our measurements in the reservoir. This gage site is part of the reservoir during most years, however, due to the current drought, it is in the river. In summer 2014, the gage at Wards Ferry will be moved to a permanent location with a cableway at the Tuolumne River at Turnback Creek.

A sampling site has been established within the upper most reaches of Don Pedro Reservoir approximately 2 miles below the current gage. The sampling site is within a relatively narrow canyon of the reservoir and has sufficient infrastructure, easy access and support from reservoir operators to collect high quality data. Water quality parameters that are being monitored continuously are temperature, turbidity, dissolved oxygen, specific conductivity, pH, chlorophyll-a, FDOM, nitrate, and particle size.

Depth and width integrated discrete water samples are also being collected during routine service visits and during rainfall and snowmelt runoff events. Discrete samples will be analyzed for suspended sediment concentrations and size, and optical analysis (absorbance and fluorescence). This will be used for the field validation of sensor measurements and the development of surrogate relationships for suspended sediment concentration, as well as a subset of metals and other constituents of concern for drinking water quality.

Once surrogate relationships are developed for suspended sediment and for a subset of metals and other constituents of concern for drinking water quality, they will be combined with flow data from the Ward's Ferry gage, used compute fluxes of the various constituents from the Rim fire burn area into Don Pedro Reservoir. The flux time series will also be analyzed, along with rain gage data, to investigate the rainfall-runoff characteristics of the watershed post-burn.

This project addresses natural hazards and climate change priorities in the USGS Science Plan (U.S. Geological Survey 2007). Understanding the effects of forest fire on hydrology and water quality—particularly in the western U.S.—will continue to be critical given the impacts on drinking water systems and projected changes in the severity and frequency of burns. In addition, this study will provide baseline data that can be used by future ecological and modeling studies to evaluate the impacts of forest fires on ecosystem health, water flow paths and water delivery.