Project Description

Introduction

For many years, Mud Lake, which is located at the north end of Bear Lake, has acted as a sedimentation basin for Bear River water diverted into Bear Lake. Bear River water, which is generally high in sediment content, is diverted into Mud Lake and then eventually into Bear Lake where it is stored for future agricultural water use. Under the current and historic flow regime of this water through Mud Lake, much of the sediment from the inflowing Bear River water has settled out prior to its flowing into Bear Lake. Over the past 100 years, however, sediment deposition in Mud Lake has resulted in channelization and other effects within Mud Lake, which have resulted in reduced filtration capacity and other impacts to the aquatic ecosystem. The purpose of this project was to begin continuous water quality monitoring at four locations around Mud Lake to provide data that can be used to assess the overall sediment and nutrient budget for Mud Lake and to evaluate the role of Mud Lake in controlling sediment and nutrient loads to Bear Lake and the Bear River downstream of Mud Lake. This study has provided much needed data that will allow managers to better manage the system using a multiple benefits approach – managing Mud Lake so that it continues to be a sediment and nutrient trap for Bear River water while supporting the unique and important aquatic ecosystem.

Project Narrative

A study was undertaken in 1985 and 1986 to estimate the amount of sediment and nutrients entering and passing through Mud Lake and the Bear Lake National Wildlife Refuge each year and to evaluate the potential effects on fish and wildlife production associated with using the marsh as a sediment and nutrient trap (Bjornn et al., 1989). This study provided a snapshot of the role of Mud Lake as a settling basin for nutrients and sediments contributed by diverted Bear River Water and provided projections for loads well into the future. However, it did not leave managers with a way to monitor, over time, potential changes in the way the system behaves based on differences in season, climate, or differences in management from year to year - or a way to evaluate the projections made in the original study. This project was aimed at creating the monitoring infrastructure needed to more accurately assess sediment and nutrient loads entering and leaving Mud Lake, and providing managers with the data and information needed to better understand how Mud Lake affects the sediment and nutrient loads within the system. We have installed continuous monitoring equipment at 4 sites around the periphery of Mud Lake to monitor the water quality of its major inflows and outflows. These sites include: 1) the Rainbow Inlet Canal where it flows into Mud Lake; 2) the causeway at the north end of Bear Lake where Bear River Water enters Bear Lake from Mud Lake; 3) the Lifton pumping station where water is pumped out of Bear Lake; and 4) the Outlet Canal where water from Bear Lake and Mud Lake leaves the complex and flows North and back into the Bear River. Continuous flow measurements at these locations have been provided by PacifiCorp Energy using existing equipment at those sites.

In addition to the continuous flow and turbidity monitoring, periodic water quality samples have been collected and sent to a laboratory for chemical analysis. These samples were be analyzed for total suspended solids and nutrient concentrations, and the results were used to establish relationships between continuously monitored turbidity and other water quality constituents. Once these relationships were generated, we evaluated the magnitude of sediment and nutrient loading at each of the four sites and investigated changes and patterns apparent in the data. We collected information from the managers of the Mud Lake system that indicate what types of management scenarios were carried out during the sampling periods to investigate changes in loading that are related to changes in the management of the Mud Lake system. To the extent possible, we investigated the effects of seasonality on the management and behavior of the system.

Scientific Questions and Project Objectives

This project was geared toward answering the following major science questions:

  1. What is the nutrient and sediment budget for Mud Lake?
  2. How does management of Mud Lake affect flow pathways and residence times within the system?
  3. Does the management of Mud Lake have a significant impact on sediment loading into Bear Lake and conversely from Bear Lake and Mud Lake to the Bear River as water is released for agricultural purposes from Bear lake?
  4. What are other exogenous factors that potentially affect these processes?