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The Flathead River flood (above Flathead Lake) of 1933 was the last great flood before Kerr Dam. The hydrograph is natural, with a peak around the beginning of June.
The flood of 1948 was the last great flood after Kerr Dam and before Hungry Horse Dam, which impounds the South Fork of the Flathead, and thus controls approximately 30 percent of the inflow to Flathead Lake. The hydrograph no longer is natural, and the drawdown from October through April is fairly steady.
The flood of 1964 occurred a decade after Hungry Horse dam began operations, producing the highest river peak on record. Although this was the highest crest on the river, it did not produce the highest peak on the lake because the duration of the flood was shorter, and thus the total volume of water less, than in 1933 or 1948 (see the graph below). The 1964 hydrograph for the lake resembles the hydrograph of 1948, but differs in that the lake was held above 2890 well into January. High lake levels in fall and winter can result in shoreline damage and erosion during heavy storms.
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