Title

The Persistence of Losing and Gaining Stream Reaches

Publication Date

8-2013

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Hydrologic Sciences

Department

Geosciences

Major Advisor

James P. McNamara, Ph.D.

Abstract

Streams are commonly classified as "gaining" or "losing" according to their interactions with groundwater. However, the gaining and losing nature of a stream can vary in space and time. Understanding the spatial and temporal persistence of gaining and losing reaches is essential for understanding controls on stream health. This is particularly important during low flow conditions when water demand is high, affecting water resources and its users in the region as well as the ecological impacts within the stream and surrounding ecosystems. Hydrologic models are typically designed to simulate high flows, and are challenged by low flows. This problem will become more important as hydrologists are asked to forecast the increasing occurrence of low flows as a consequence of climate change. Low flow discharges have been a focus of research as they are occurring earlier in the summer and have longer durations throughout the fall. This study investigated the gaining and losing nature of a stream as well as the particular timing of gaining/losing transitions in a small watershed in Southwestern, Idaho during the 2012 calendar year.

To understand the spatial and temporal changes occurring, chloride was used as a chemical tracer to measure discharge and monitor gains and losses by mass recovery in a 1200 m study reach. The study reach located between two gauging stations (2500 m apart) was divided into six 200 m sub-reaches where conductivity probes were placed to perform sequential multi-slug tracer injections at least once a month. Discharge results between slug injections and gauging stations did not always agree in magnitude, but did however show similar trends during both gaining and losing times. Also, the transition from gaining to losing conditions in late June was also captured by both of these methods. Although both methods of stream measurement agreed with one another, variations in streamflow were observed at the 200 m scale during the winter and spring. These variations were investigated and were contributed to connecting tributaries and hillslope springs, which were active from snowmelt and spring precipitation. During the summer recession, losing conditions were observed at all spatial scales and discharge measurements were the lowest at the beginning of August. From here on into the fall, discharge increased with continued losses, even though no significant precipitation occurred until the second week of October. I think the system is greatly affected by riparian vegetation, which resulted in the June transition as well as the increase in summer discharge when no other hydrologic controls were affecting streamflow.

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