Site establishment for large rivers water quality monitoring: stage-discharge relationships at selected sites within the St. Croix National Scenic Riverway

Magdalene, Suzanne

Long-term monitoring of river water quality requires an understanding and analysis of river discharge. Because water quality concentrations are affected by fluctuations in flow, a record of discharge enables the assessment of flow-corrected concentrations and loading analysis. For example, a suspended sediment concentration of 1000 mg/L at low flow would represent something entirely different than the same value at high or flood flows. For this reason, the qualitative estimate or quantitative measurement of discharge is recommended by the National Park Service – Water Resources Division (NPS-WRD) for all freshwater flowing waterbodies during each sampling event (Penoyer 2003). It is preferable to conduct long-term monitoring at or near a continuous flow gaging station. In the absence of any nearby continuous recording gaging station, discharge can be measured directly or calculated from discharge rating curve relationships. A rating curve is the relationship between stage height and discharge at a particular cross-section of a river. As follow-up to the development of a protocol for monitoring the water quality of large rivers within the NPS-Great Lakes Network (GLKN) (Magdalene et al. 2007), this project sought to establish stage-discharge rating curves for monitoring sites within the St. Croix National Scenic Riverway (SACN) that do not have nearby USGS flow gages. Preliminary work (summarized in Table 1) identified three monitoring sites that needed additional flow monitoring: on the Namekagon River at Earl, on the St. Croix River at Norway Point, and on the Snake River at the Chengwatana State Forest Campground. The purposes of this project were: 1) to gather concurrent water level (stage) and water flow (discharge) information over a range of flows at these sites during field work conducted May-August 2006, and 2) to analyze the data for stage discharge rating relationships and develop rating equations. This document summarizes the tasks of site establishment including quality assurance measures, especially those that focus on developing stage-discharge ratings: measuring stage, measuring discharge, and rating curves. Particular attention is paid to the stage and discharge methods that are most applicable to the range of flow conditions expected for SACN. Average annual river flows within SACN range from 480 cfs (1928-1970, 1988-2006) at Trego (USGS gage #05332500) near the upstream end, to 4,380 cfs (1911-2006) at St. Croix Falls (USGS gage #05340500) toward the downstream end. The three flow monitoring sites required a range of methods depending on flow volumes, from wading to boating to working from a bridgedeck. The rating curve development process for each of the three monitoring sites is fully documented, and this report includes recommendations for adjustments to the on-going collection of flow data at NPS-GLKN large river sites.

Site establishment for large rivers water quality monitoring: stage-discharge relationships at selected sites within the St. Croix National Scenic Riverway

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