| Résumé | (Sommaire disponible en anglais seulement)
Continuously-recording water-level gages in Rainy Lake and Namakan Reservoir are used to regulate water levels according to the 2000 Rule Curves
established by the International Joint Commission. However, gage water levels were established using a variety of vertical datums, confounding efforts to model the flow of water through the system, regulate water levels during periods of high inflow,
and evaluate the effectiveness of the 2000 Rule Curves. In October 2014, the U.S. Geological Survey, Natural Resources Canada, and Environment Canada began a joint field study with the goal of obtaining precise elevations referenced to a uniform
vertical datum for all reference marks used to set water-level gages throughout Rainy Lake and Namakan Reservoir. Three field crews deployed global navigation satellite system (GNSS) receivers statically over 17 reference marks co-located with active
and discontinued water-level gages throughout Rainy Lake, Namakan Reservoir, and selected rivers. A GNSS receiver also was deployed statically over a National
Geodetic Survey cooperative base network control station that was used as a
quality-control reference mark, resulting in a total of 18 surveyed reference marks in this study. Satellite data were collected simultaneously over a five-day period and processed to obtain accurate positioning and elevations for surveyed reference
marks. Processed satellite data were used to convert published water levels to elevations referenced to the Canadian Vertical Datum of 2013 (CGVD2013) in order to compare watersurface elevations referencing a uniform vertical datum throughout the
study area. In this report, an “offset” will refer to the correction that needs to be applied to published data from a particular gage to produce elevation data referenced to CGVD2013. Offsets were applied to water-level data from surveyed gages to
further evaluate the accuracy and utility of updated reference mark elevations presented in this study. Mean water levels during the survey period for active gages surveyed in this study were converted to water-surface elevations iii referenced to
CGVD2013, and graphical comparisons of water-surface elevations for gages in Namakan Reservoir, Rainy Lake, and selected rivers are presented in this report. Four surveyed gages in Namakan Reservoir were discontinued in October 2012, so published
mean daily water-levels (May-September 2012) were used to calculate mean monthly water levels in order to compare all gages in Namakan
Reservoir. Mean monthly water levels were converted to water-surface elevations in CGVD2013, and a graphical
comparison of water-surface elevations among gages is presented in this report. Key findings: 1. Precise elevations referenced to CGVD2013 were obtained for 16 reference marks used to establish water levels at active and discontinued gages in the
study area 2. A precise elevation referenced to CGVD2013 was not obtained for one discontinued gage (map number 12; station number 462626092302001) because the data card was damaged during retrieval 3. Elevation data obtained for an NGS cooperative
base network control station of known elevation
closely matched elevation data published in 1996 4. Elevation data were used to develop offsets that correct previously-published gage data to reference a uniform vertical datum (CGVD2013) 5.
Comparison of previous and corrected elevations among active gages in Namakan Reservoir indicate improvements in the accuracy of water-elevation data as a result of this study 6. Corrected water-elevation data collected from active and discontinued
gages in Namakan Reservoir during 2012 identified hydrologic discrepancies at two discontinued gages (map numbers 11 and 13, station numbers 482611092483801 and 482147092291701) 7. Caution should be used when applying presented gage offsets to
discontinued gages 8. Additional surveys may help to (a) resolve hydrologic discrepancies at two discontinued gages and (b) develop an offset for a discontinued gage for which data could not be obtained in this study iv Potential Applications of Key
Findings: The precision and accuracy of the elevation data presented in this study are suitable for several management applications in the Rainy River Basin. Water-level offset corrections can be used in the evaluation of the 2000 Rule Curves for
Rainy Lake and Namakan Reservoir and in flood damage curves that fully assess the benefits of one regulation approach over another. Presented water-level offsets also may provide the information needed to calibrate HEC-RAS models developed by
nvironment Canada for four narrows that connect the lakes of Namakan Reservoir. Furthermore, improved accuracy of water-surface elevations can enhance a digital elevation model being developed by Environment Canada. Finally, corrected water-surface
elevations can benefit other modeling studies designed to assess the effects of the 2000 Rule Curves on aquatic vegetation, benthic invertebrates, northern pike,
and walleye. Results of this study also indicate potential locations for additional
data collection. Collected water-surface elevations data indicate that repeating the GNSS survey at USGS temporary gages 482611092483801 and 482147092291701 (map numbers 11 and 13, respectively) may be useful in resolving observed hydrologic
discrepancies between these gages and the other gages surveyed for this
study. In addition, because the data card was damaged during retrieval of deployed field equipment, no GNSS data was obtained for USGS temporary gage 482626092302001 (map
number 12). Therefore, repeating the GNSS survey at this gage would provide the data required to develop an offset that corrects collected water-level data to elevations referenced to CGVD2013. |