Fact Sheet 081-03

August 2003

Lower Merced River and Mustang Creek Watersheds Selected for a National Water-Quality Study

The PDF for the report is 769 kb

Table of Contents


The lower Merced River and Mustang Creek watersheds and four other watershe...

At a typical study site, several methods are used to collect water and chem...


Data Collection in the lower Merced River and Mustang Creek Watersheds, 200...

The U.S. Geological Survey (USGS) is studying five watersheds across the Nation to better understand how natural factors and agricultural management practices (AMPs) affect the transport of water and chemicals. Natural factors include climate and landscape (soil type, topography, geology), and AMPs include practices related to tillage, irrigation, and chemical application. The study approach is similar in each watershed so that we can compare and contrast the results and more accurately predict conditions in other agricultural settings.

Map of study area

The lower Merced River and Mustang Creek watersheds and four other watersheds have been selected by the USGS National Water-Quality Assessment (NAWQA) Program for a special study of agricultural chemicals and water quality.

Study objectives

Why study these watersheds?

The lower Merced River and Mustang Creek watersheds, along with the other four watersheds, represent nationally important agricultural settings (chemical use, crops, and AMPs) and natural settings (climate, geology, topography, and soils). Mustang Creek is a small tributary to the Merced River, which drains into the San Joaquin River. AMPs are diverse throughout the lower Merced River watershed, which contains orchards, vineyards, row crops, dairies, and other animal operations typical of the San Joaquin Valley.

Other features of the watershed that are relevant to this study:

Cross section of a typical study site

At a typical study site, several methods are used to collect water and chemical samples from the air, soil, surface water, and ground water. After being applied to the land surface, agricultural chemicals can move upward into the atmosphere, downward through the soil to shallow ground water and underlying aquifers, eventually discharging to streams, or run off across the land into streams, eventually moving downstream to reservoirs and coastal waters. This process can take days, weeks, or even decades if water moves underground through the ground-water system.

Data Collection in the lower Merced River and Mustang Creek Watersheds, 2003–2004

What kind of data Why the data are collected How often
Meteorological data, including rainfall, wind speed, solar radiation, and air temperature. Soil temperature and moisture To determine amount of precipitation and estimate amount that reaches the water table and how much is lost to evapotranspiration Continuously for 2 years
Streamflow at 2 newly installed gaging stations within the Mustang Creek watershed. Streamflow on the Merced River measured by California Dept. of Water Resources. To interpret water-quality data correctly (the amount of water in streams affects chemical concentrations) Continuously
Quality of stream water, runoff water, rain water1 To quantify the transport and behavior of natural and agricultural chemicals Several times a year (>14 samples) for 2 years, with intensive sampling during application season
Ground-water levels in wells To determine direction of ground-water flow, which affects transport of chemicals At least quarterly in some wells, continuously in others for at least 1 year
Quality of ground water, soil water, and shallow water in and around streambed/riparian zone1 To quantify the transport and behavior of natural and agricultural chemicals At least quarterly for 1 year
Quality of sediment in streambed and soils in agricultural fields1 To quantify the storage, behavior, and transport of water and chemicals in the soils and sediment At least once during study

1In this study, water-quality and sediment-quality data include concentrations of nutrients (nitrogen and phosphorous), pesticides and pesticide breakdown products, and natural constituents and properties, including major ions (calcium, magnesium, chloride, etc.), organic carbon, dissolved oxygen, and temperature.

We appreciate your help

We are working with local growers and land owners to gain access to study sites. We also need information about the watershed and about current as well as historical agricultural management practices—past practices also affect concentrations of agricultural chemicals in ground and surface water.

We will report the findings of the study in public meetings and in publications. The findings will provide information that will be useful for improving agricultural management locally and nationally, and will guide future studies in other watersheds.

We would like to thank

East Merced Resource Conservation District

Merced River Stakeholders

Turlock Irrigation District

Merced County Dept. of Agriculture

California Central Valley Regional Water Quality Control Board

For more information

Joseph Domagalski, Lead Scientist, Lower Merced River and Mustang Creek study (916) 278-3077,

Paul Capel, Team Leader, National study (612) 625-3082,

NAWQA Program

San Joaquin-Tulare Basins NAWQA


Dubrovsky, N.M., and others, 1998, Water Quality in the San Joaquin-Tulare Basins, California, 1992-95, U.S. Geological Survey Circular 1159.

The USGS provides reliable scientific information to describe and understand the Earth; minimize loss of life and property from natural disasters; manage water, biological, energy and mineral resources; and enhance and protect our quality of life.

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