Work is underway to link variations in largescale atmospheric circulation to variations in the Merced and Tuolumne River discharge and then the variations in discharge to variations in chemistry. To do this, river and stream chemistry needs to be monitored at a sampling rate compatible with hydroclimate variables (i.e., hourly/daily). The protocols for a conservative chemical parameter, specifi c conductivity, have been established. Procedures for monitoring a non-conservative property (nitrate plus nitrite) are in progress as well as the addition of dissolved silica. The atmosphere is considered the major high elevation source of nitrogen; weathering rock and soil minerals are the major source of dissolved silica. The Water Chemistry Observation system has two confi gurations 1) Conductivity-Temperature-Pressure (CTP) and 2) CTP plus Chemical Analyzer.

Figure 1. Stylized version of a typical in situ Conductivity – Temperature – Pressure installation.

The CTP system Confi guration 1) is composed of a SeaBird microcat SB37 sensor-logger mounted on a concrete platform (Figure 1). The system is roughly 30” long by 10” wide by 10” high (photo 1). The unit is placed on a stream bed with at least 1 foot water depth and tethered to shore using a 3/16” stainless steal cable. With judicious placement at or near bridge abutments in the shadow of the bridge, even in the clear waters of YNP, the unit is nearly invisible. To allow access to the data stored by the instrument without removing it from the stream a 3/8” black cable extends to the shore and is terminated in a gray water proof lock box. The cable and box are camoufl aged using local boulders and/or mounted on the bridge superstructure to minimize visual impact.

Photo 1. Tenaya Creek CTP

Confi guration 2), Happy Isles gage and the Power Station - include a streamside analysis system (WS Oceans NAS2) in addition to the CTP. The analysis system pumps water from the Merced River hourly and at present analyses for nitrate plus nitrite. The volume of water required for the analysis is small (< 20ml) and no chemicals are returned to the stream. The unit is housed in a 4'x3'x4' box (photo 3). A 1⁄2” gray conduit extends from the box into the stream protecting the sample tube from sunlight and water turbulence. Due to the lack of power at the Happy Isles site a 4'x3' solar panel is used to prevent freezing during the cold season.

Photo 3. Happy Isles Analyzer Box and CTP locations.