The Army Corps of Engineers investigated Puget Sound for military defense purposes and chose Lake Washington as the best location for a Naval base, due to the security of the waters, proximity to supplies of fresh water, coal and timber, and the less corrosive effect on wooden ship hulls of a fresh water port. A report by Lieutenant Thomas H. Hardy of October 13, 1871, forwarded to Congress by Army General Barton S. Alexander of the Board of Engineers of the Pacific Coast said the coal fields of Seattle Coal and Transportation Company, two miles east of Lake Sammamish, supplied 1,500 tons per month of steam ship grade coal, and potentially twice this amount. The route directly across the narrowest part of Seattle, that is, Semple's Canal from Leschi straight across to present-day Harbor Island, was rejected in this report because of the 200 to 300 foot height of the hills that would have to be cut through. The route via the Black and Duwamish Rivers would be crooked and several miles in length, would let out into shoals rather than deep water, and would have to be frequently dredged to remove sand brought by river flooding. As to connecting Lake Union to Shilshole Bay—the route ultimately chosen—Gen. Alexander had "serious objections", including the expense and the need for dredging a channel, and that the canal line let out in shoal water that was exposed to heavy seas, and would be less defensible in wartime, being "exposed to the cannonade of an enemy". Alexander was less opposed to digging straight south from Salmon Bay through Interbay to Smith Cove, but would still terminate in shoals and be just as costly as his preferred route from Lake Union through Mercer's Farm into Elliott Bay, while being less defensible. The estimated cost for the project was $4.7 million.