1959 Flood

Brian:
Do you have an Indian Ford Pool reading for April 1959?
Tom
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Tom - 

I do not....I would have to research that for you.

Brian

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There are no official records for water levels on Lake Koshkonong prior to 1987, when the USGS gage was installed on Bingham point. So we have to take a look at discharge in the Rock River and the correlation between discharge in the river and the elevation of Lake Koshkonong to estimate the stage on Lake Koshkonong for April 1959.

To give you the bottom first, I estimate that the April 1959 flood had about 73% of the discharge of the 2008 flood, and probably reached a Lake Koshkonong stage of approximately 13.0 feet, gauge height, or 783.0 according to the "old" NGVD 1929 elevation datum.  As you know, the 2008 flood reached a gauge height of approximately 15.1 – meaning that the April 1959 flood was probably about 2 feet lower in height than the 2008 flood.

The Indianford dam gauge was installed in May 1975, so that won't help define how big the 1959 flood was. The USGS Watertown gauge was operating in the 1950s, but so far upstream that it might not be representative of the discharge that was going to Lake Koshkonong. However, the Rock River at Afton gauge between Janesville and Beloit has a more similar watershed area to Koshkonong and the Afton gauge has been in operation from 1914.

The peak daily discharge at Afton was 12,100 CFS on April 10, 1959.  The peak daily discharge for the 2008 flood at Afton was 16,500 CFS on June 22, 2008. The peak discharge at Indianford dam was 14,800 CFS, also on June 22. So the 1959 flood was large, but the 1959 flood discharge was only around 73% (12,100 / 16,500) of the 2008 flood discharge.

To estimate the Lake Koshkonong elevation for the April 1959 flood, we can make the assumption that the hydraulics of the Rock River have not changed much at high flood discharge (probably a good assumption) and the ratio of the 1959 to 2008 flood at Indianford was about the same as it was at Afton (probably a reasonably good assumption considering the size the watersheds). So our estimate of the 1959 flood at Indianford is 73% of the 2008 flood, or 10,800 CFS.  The stage on Lake Koshkonong that corresponds to 10,800 CFS is approximately 13.0 gauge height, based on the USGS discharge records at Indianford and the gauge records for the Lake. 13.0 gauge height corresponds to 783.0 using the "old" NGVD 1929 datum.

 

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