|Report / Graph Revisions|
June 15, 2009 - Statistical Level/Flow Data Updated
The water level and flow statistical percentile data on the reports and graphs have been updated to the 30 year period 1976 to 2005. Previously the reference period was 1970-1999. As with the previous period, some percentile data have been based on simulation model results to account for the impacts of the new rule curves on Rainy and Namakan lakes, which were implemented by the International Joint Commission on January 6, 2000. The data affected are the percentile data for Namakan Lake outflows and Rainy Lake inflows & outflows. Lake of the Woods inflows are not affected, although they were in the 1970-1999 period.
The data at some sites have also been extended. For example, the Board's records for water levels at Manitou Rapids on the Rainy River previously began in 1983 but have now been extended back to 1928. The periods of record referenced for percentiles, maximums and minimums for levels or flows at particular sites can be seen on the percentile tables available at www.lwcb.ca/percentiles/.
May 13, 2005 - Lake of the Woods Mean Level Computation Revised
The mean water level for Lake of the Woods is now computed as the average of the gauge readings at Springsteel Island, Hanson Bay, Cyclone Island and Clearwater Bay. Previously the mean level was computed as the average of only the Clearwater Bay, Cyclone Island and Hanson Bay gauge readings. In addition, a datum correction has been made for the Clearwater Bay gauge.
There are currently six water level gauges on Lake of the Woods: at Kenora (Keewatin) at the north end near the outlet, at Clearwater Bay to the northwest, at Cyclone Island near the Northwest Angle, at Hanson Bay to the southeast, and two in the southwest corner near Warroad, one on the Warroad River and one at Springsteel Island (the latter was intended to replace the Warroad River gauge). By averaging the level at several different locations to determine the mean lake level, the Board is attempting to get the most representative level of the lake, without wind or wave effects. The Keewatin gauge is not used in the average as the level of the bay near the town becomes lower than the main lake as lake outflow increases, due to currents through restricted channels near the north end.
The Warroad gauge was used in the average prior to the early 1980's (before the establishment of the Springsteel Island gauge), but was dropped due to an unexplained discrepancy. For years levels at this gauge had tracked in line with those at the other gauges but, over the course of one winter, shifted upwards relative to the others while the others maintained their normal relationship. The cause of this shift was only recently found to be due to a datum adjustment at a local benchmark, to which both the Warroad and Springsteel gauges are tied. Both gauges generally indicate similar water levels, but the Warroad gauge is at times affected by flows in the Warroad River.
As part of the gauge discrepancy investigation, the benchmark histories for the other gauges were reviewed as well, and a possible problem was found with the Clearwater Bay gauge. Survey work conducted in early 2005 revealed that the Clearwater Bay datum had been set too high when the gauge was first established.
The net effect of adding the Springsteel levels (which tend to be higher) to the average, coupled with the Clearwater correction (now reads lower), is that the mean lake level is very little changed from the previous calculation. However, the difference in readings from the south end to the north end of the lake is greater. Whereas, in the past, there was no explanation for this difference (apart from datum and survey errors), it is now known that the majority of the difference is probably due to "isostatic rebound". The land mass is still rebounding since the weight of the glaciers was removed, but at a differential rate, with the north end of the lake rising faster than the south end. Studies have shown the north end to be rising about 10 cm per century more than the south end.
Until such time as an adjustment for the isostatic rebound may be made, including gauges at both ends of the lake in the calculation of the mean level minimizes the effect of the rebound in determining a representative lake level. Also, although water level readings tend to be more volatile at Springsteel due to wind and wave action on Big Traverse Bay, the inclusion of the fourth gauge in the average does render a little more stable the calculation of lake inflows.
October 1, 2000 - Statistical Level/Flow Data Updated
The water level and flow statistical percentile data on the reports and graphs have been updated to the 30 year period 1970 to 1999. Previously the reference period was 1963-1992. In addition, some percentile data have been based on simulation model results to account for the impacts of the new rule curves on Rainy and Namakan lakes, which were implemented by the International Joint Commission on January 6, 2000. The data affected are the percentile data for Namakan Lake outflows, Rainy Lake inflows & outflows, and Lake of the Woods inflows. Lastly, the percentile data used in the reports and shown on the graphs are now computed on a ¼-month basis rather than ½-month.
The change from ½-month to ¼-month percentiles and reporting periods should more accurately
reflect variability and trends in levels and flows. While there may be more volatility in
inflows, especially for the larger lakes, changing trends in current inflows should be
identified sooner. The actual elevation data are still tabulated and plotted daily as
before, but are now compared to the new ¼-monthly percentiles for historical comparison.
The actual inflows used for comparison to historic are now 7-day averages instead of 14-day
averages. On the Weekly and Daily Reports, the "14 Median / %ile" columns for both outflows
and inflows have been replaced with "7 Median / %ile" columns. On the graphs, 7-day average
inflows are now plotted daily against the ¼-monthly inflow %iles, which are plotted
¼-monthly. Outflow data are still plotted daily; the outflow %iles are ¼-monthly outflow
average values plotted ¼-monthly.