2. Field Site- San Joaquin River (SJR)-Merced River confluence
In 1991 through 2004, the San Joaquin-Tulare basins became a study site of the
National Water-Quality Assessment (NAWQA) program. Studies of the water usage in
the region tally agricultural use at over 90 percent whereas urban use remains below 10
percent (Ref. 2.1- Dubrovsky 1998, Ref. 2.2- Burow 2004). Such heavy agricultural use
of water and eventual recharge from irrigation back to the water supply cause known
increases in concentrations of pesticides and nutrients that can have adverse effects on
aquatic and sediment organisms as well as drinking water supplies (Ref. 2.1, Ref. 2.3Gronberg 1998).
In this work, the area of interest is the where the Merced River intersects with the
San Joaquin River (SJR). The Merced River forms its own sub-basin which is located on
the east part of the SJR Basin (Ref. 2.4- Gronberg 2004). The river bed sediments near
the site location were classified as medium-to-coarse sand and silty clay layers in the
shallow subsurface (Ref. 2.5- Zamora 2008). By the lower parts of the Merced River,
near our study site, agricultural land usage is roughly 55-56 percent (Ref. 2.4, Ref. 2.5),
and agricultural inputs are expected. The SJR, however, intakes distinguishable amounts
of nutrients right before the study site from the Mud and Salt Slough, especially nitrate
where half the load are due to these sloughs. Figure 2.1. shows a regional map including
the sloughs and a waterway flow diagram with abbreviated names of USGS gauging
stations. From Figure 2.2, the turbidity from the SJR is greater than that of the Merced
River by eye. Ultimately, these differences as well as others serve as tracers in
distinguishing one river from another.
From August 7th to August 12th in 2007, extensive cross-sectional measurements
for water quality and flow were taken where the SJR meets the Merced River, which will
be commonly referred to as: SJR-Merced confluence, in this document. Three crosssectional data sets are used in this work. The first cross-section, or transect (T1), was
located next to the USGS monitoring station at Newman, denoted as NEW. T1 is
roughly 290 meters downstream of the confluence, estimated by Google Earth (Ref. 2.7Amaya 2007). The second transect (T2) is located roughly 115 meters- estimated by
Google Maps- upstream of the confluence on the Merced River. The third transect (T3)
is located roughly 85 meters upstream of the confluence on the SJR. Figure 2.2 below
shows a satellite image generated from Google Maps and edited using Paint.NET with
transect locations and denoting the direction of flow.
Figure 2.1: Regional map with flow diagram and abbreviated gauging station names are in light green
circles (Ref 2.6- Fisher submitted 2008).
Figure 2.2.: Zoomed in figure showing transect locations and denoting flow of the SJR and Merced River
from Google Maps and edited with Paint.NET. Note that the satellite image was not taken at the same time
of the study. Therefore, water levels will be different.
2.1
N.M. Dubrovsky, C.R. Kratzer, L.R. Brown, J.M. Gronberg, and K.R. Burow.
Water Quality in the San Joaquin-Tulare Basins- California, 1992-95. USGS
Circular 1159. 1998.
2.2
K.R. Burow, J.L. Shelton, J.A. Hevesi, and G.S. Weissmann. Hydrogeologic
Characterization of the Modesto Area, San Joaquin Valley, California. USGS
Scientific Investigations Report 2004-5232. 2004.
2.3
J.A.M. Gronberg, N.M. Dubrovsky, C.R. Kratzer, J.L. Domagalski, L.R. Brown,
and K.R. Burow. Environmental Setting of the San Joaquin-Tulare Basins,
Califronia. USGS Water-Resources Investigations Report 97-4205. 1998.
2.4
J.A.M. Gronberg, C.R. Kratzer, K.R. Burow, J.L. Domagalski, and S.P. Phillips.
Water-Quality Assessment of the San Joaquin-Tulare Basins- Entering a New
Decade. USGS Fact Sheet 2004-3012. 2004.
2.5
C. Zamora. Estimating Water Fluxes Across the Sediment-Water Interface in the
Lower Merced River, California. USGS Scientific Investigations Report 20075216. 2008.
2.6
J.C. Fisher, H. Pai, S.V. Amaya, C. Butler, and T.C. Harmon. Precision Flow
and Salinity Mass Balance Assessments across the Merced-San Joaquin River
Confluence Zone. Water Resources Research. (In review).
2.7
S.R.V. Amaya. Descriptive Analysis of the Turbulent Velocity Profiles for Two
Cross Sections in a River Confluence Zone. UC Merced Thesis. 2007.