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Methods for Estimating Daily Upstream Location of the Freshwater-Saltwater Interface along the Maurice and Cohansey Rivers, New Jersey

Scientific Investigations Report 2025-5090
Next Generation Water Observing Systems
By: , , and 
https://doi.org/10.3133/sir20255090

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Abstract

The Delaware River basin (DRB) provides drinking water to 15 million people in the surrounding area. Water is frequently withdrawn from the freshwater reaches of streams, above head of tide, in the DRB for use as public drinking water. During extended periods of low flow, saltwater can move upstream, which can threaten drinking-water supplies in the basin. Due to spatial patterns in bathymetry, tidal influences within the DRB, and varying weather conditions, it can be hard to predict the movement and upstream extent of the freshwater-saltwater interface, often defined as the salt-front. Although there is a relationship that predicts this location in the main stem of the Delaware River, there lacks a relationship for its tributaries, such as the Maurice and Cohansey Rivers in southwestern New Jersey. In this study, a relationship was developed between daily specific conductance (SC) at gage locations along the tidal river reaches of the Maurice and Cohansey Rivers to the daily upstream location of the salt-front. The study augmented existing real-time tide gage data with the collection of water temperature and specific conductance data to develop the relationship. Additionally, longitudinal profiles upstream of the selected tide gages were conducted during a range of high tide conditions to define the location of the salt-front. Equations were then developed that related the daily SC measured at the tide gage to the upstream location of the salt-front. The equations were used to estimate the daily upstream location of the salt-front for the period of July 15, 2021, to July 15, 2024. This work can aid in understanding the propagation of saltwater upstream, which can affect local communities and crop farmers along these tidal reaches of the DRB.

Plain Language Summary

The Delaware River basin serves as the drinking water supply for 15 million people. However, saltwater can move upstream during low- flow periods, threatening this water supply. The study established a relationship between daily specific conductance and the position of the salt- front. Data were collected from existing U.S. Geological Survey tide gages and multiparameter water- quality sondes installed in 2021 to record specific conductance and water temperature. For the Maurice River, the salt- front varied from around 10.8 to 23.2 river miles over the study period. For the Cohansey River, it ranged between 18.5 to 20.4 river miles. The position of the salt- front depended on freshwater discharge from rainfall and tidal patterns. Low freshwater flows led to the salt- front moving upstream.

Suggested Citation

Closson, J.L., Suro, T.P., and Niemoczynski, L.M., 2026, Methods for estimating daily upstream location of the freshwater-saltwater interface along the Maurice and Cohansey Rivers, New Jersey: U.S. Geological Survey Scientific Investigations Report 2025–5090, 19 p., https://doi.org/10.3133/sir20255090.

ISSN: 2328-0328 (online)

Study Area

Table of Contents

  • Abstract
  • Plain Language Summary
  • Introduction
  • Purpose and Scope
  • Study Area
  • Methods of Data Collection
  • Measuring the Location of the Salt-Front
  • Developing Methods for Estimating the Upstream Location of the Salt-Front
  • Analysis of Estimated Salt-Front Location in the Maurice and Cohansey Rivers
  • Estimating Average Chloride Concentration and Specific Conductance
  • Summary
  • Acknowledgments
  • References Cited
Publication type Report
Publication Subtype USGS Numbered Series
Title Methods for estimating daily upstream location of the freshwater- saltwater interface along the Maurice and Cohansey Rivers, New Jersey
Series title Scientific Investigations Report
Series number 2025-5090
DOI 10.3133/sir20255090
Publication Date April 02, 2026
Year Published 2026
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) New Jersey Water Science Center
Description Report: v, 19 p.; Data Release
Country United States
State New Jersey
Other Geospatial Maurice and Cohansey Rivers
Online Only (Y/N) Y
Additional Online Files (Y/N) N
Additional publication details