It has been pointed out that several features or bends of the American River are not illustrated. I don’t believe it was the intent of the map maker to accurately depict river but to note its relative position to the mines. The distances of 25 miles to the lower mines and of 50 miles to the upper mines is pretty accurate. The lower mines were also known as Mormon Island for the first group of miners who did extensive mining after the initial discovery of gold by Marshall. The distance to the upper mines, site of Marshall’s gold discovery is also relatively accurate considering hilly terrain that had to be traversed to get to the location.
Within the 90 foot elevation change of the lake, it was proposed that all standing and down timber, brush over 6 feet high or with trunks greater than 2 inches in diameter would be cleared out. Trees whose height reached to 360 feet in elevation would be topped to 10 feet below the expected low water elevation.
By the time of the Army Corp report conducted its inventory most of the North Fork Ditch had been lined with concrete. Many of the appurtenances were also concrete such as wasteways, intake structures and sluice gates. Of the 37 flumes, 32 were constructed of timber and only 5 were metal. The timber flume construction allowed them to be built with small changes or bends in the direction to navigate around boulders and hillsides. The metal flumes, by contrast, were best adapted to spanning a small ravine in a straight line.
In 1849 U. S. Army Lieutenant George H. Derby performed topographical survey of the Sacramento Valley. The “Topographical Memoir Accompanying Maps of the Sacramento Valley, &c.” was found in Quarterly of the California Historical Society Vol. XI No. 2 publication dated June 1932. I found the small quarterly report in a book store in San Francisco and was attracted to it because of the inset map of the Sacramento Valley. The map is a reproduction of Lieutenant Derby’s topographical map he made for his report.