RAINFALL AND RUNOFF. The mean annual rainfall is about 28.5 inches of which 3 inches are precipitated in the form of snow. The nearest long time record is at Minneapolis and is continuous from 1866. The wettest year was 1868 when the rainfall was 41.6 inches, and the driest 1910, when the precipitation was 11.6 inches. Runoff records of Rum River have been maintained continuously since 1909. These show a runoff ranging from 1.87 to 2.41 inches or from 6.7 to 16.9 per cent of the rainfall. REGULATION OF FLOW. Lake Mille Lacs forms a natural reservoir for Rum River, tending to equalize its flow. In years of very low flow, however, this regulation is detrimental to the river, as of the area at the outlet more than half is comprised within the lake surface itself, where evaporation exceeds precipitation by 6,inches or more. This loss must be made up from the small tributary. runoff. The effect of this is seen in the period of low rainfall from the spring of 1910 to the summer of 1911. The runoff at Onamia, which included not only Lake Mille Lacs but the three small lakes below its outlet, was very small and ceased entirely during the winter. During the greater portion of this period there was said to be no flow from. Mille Lacs, the flow past Onamia representing the area below the lake. DRAINAGE WORK Owing to the general flatness of portions of the basin, drainage is defective. The following table compiled from the Report of the State Drainage Commission shows the need for improved drainage and the work already accomplished: DRAINAGE AREAS. The following drainage areas have been measured in the basin of Rum River: Location. GAGING STATION RECORDS. RUM RIVER AT ONAMIA, At the steel highway bridge at Onamia, 200 yards below the outlet of Lake Onamia and 5 miles above the mouth of Bradbury Brook. Records available.-September 24, 1909, to December 31, 1912. Drainage area. -414 square miles, of which 207 square miles are taken up by the water surface of Mille Lacs Lake. Gage. -Vertical staff. The gage was located originally at the wooden highway bridge just below the Soo Railway bridge, but May 4, 1910, this bridge was destroyed and the gage moved 200 yards downstream to the steel highway bridge. The new gage was set to read the same as the old one. Channel. Shifting, affected by grass and high water. Discharge measurements.-Made from the steel highway bridge. Regulation. Two miles below Onamia is an abandoned logging dam which raises the water level about 3 feet but does not control the flow. As there is a good fall to the river, the influence of this dam does not reach the gaging station. Owing to the natural storage afforded by the lakes, the range of stage at Onamia is slight. Winter flow. At the original location gage heights were practically unaffected by ice. At the present location, however, the river freezes over. Accuracy. The extremely low water of 1911 was favorable to the growth of grass in the river channel. This growth created an appreciable backwater at the gage and therefore the records for that period are not as reliable as during the remainder of the period. Day. 1909. Daily discharge, in second-feet, of Rum River at Onamia. Jan. Feb. Mar. Apr. May. June. July. Aug. Sept. Oct. Nov. Dec. |