The kingpin structure of the American scheme to construct a ship canal across Panama in 1907–1914 was an earthen dam of unprecedented scale and scope at Gatun, to retain the aggregate flow of the Chagres River and its principal tributaries. Upon this structure alone, the entire plan rested, because it created the man-made lake rising 85 ft (25.9 m) above sea level, which allowed ships to cross the 550 ft (167.6 m) continental divide between the Atlantic and Pacific Oceans. Everything about Gatun Dam was enormous. Its dimensions were without precedent: a crest length of 8,200 ft (2,500 m) and a maximum width of 2,300 ft (701 m). With a height of 105 ft (32 m) above sea level, it stored sufficient water to maintain an operating pool covering 164 mi2 (425 km2). At its center was the most critical structure, a mass concrete spillway capable of passing flood flows of the unpredictable Chagres River. The biggest problem with the site was the underlying geology, which included two deepley incised paleo-channels. The massive embankments were placed over these paleo-channels, which were up to 258 ft (78.6 m) deep. The channel infill of the upper 50 ft (15.2 m) was of relatively low permeability, mostly sandy silts and clay. There were more pervious sands and gravel lying beneath these, which allowed deep seepage cutoffs to be precluded.