So much for the place. As to the time, the choice was even more limited. The tidal range varied from an average spring tide[66] height of 23 feet to an occasional maximum of 33 feet. Landing craft required a tide of 25 feet to navigate the mud flats of the harbor, and the LSTs must have 29 feet. Only during a few days in the middle of September and October were those depths provided by spring tides of the next 12 weeks. MacArthur rejected an October date as being too late in the season, so that 15 September became D-day by virtue of elimination. [66] A spring tide is a higher than normal tide caused by the sun and moon being in conjunction or opposition, as at new moon and full moon. Conversely, when the moon is at first or third quarter the tide (neap tide) is smaller than usual. A late afternoon H-hour was also a choice of necessity. Islands, reefs, and shoals restricted the approach to the outer harbor, and currents ranging from three to six knots multiplied the chances of confusion. This meant that daylight landings were necessary for all but small groups. Much of the inner harbor was a vast swamp at low water, penetrated by a single dredged channel 12 to 13 feet deep.[67] The duration of spring tides above the prescribed minimum depth averaged about three hours, and during this interval the maximum in troops and supplies must be put ashore. Every minute counted, since initial landing forces could not be reinforced or supplied until the next high water period. [67] JANIS No. 75, ch. V. Time and tide seemed to have combined forces to protect Inchon from seaborne foes. As if such natural obstacles were not enough, the target area provided others. Two islands, Wolmi-do and Sowolmi-do, located in a commanding position between the inner and outer harbors, were linked to each other and to Inchon by a causeway. In advance of intelligence reports, it must be assumed that rocky, wooded Wolmi-do would be honeycombed with hidden emplacements for enough guns to create a serious menace for the landing craft. This critical terrain feature must somehow be reduced as a preliminary to the main landing during the high tide of late afternoon. Inchon being situated on a hilly promontory, the “beaches” were mere narrow strips of urban waterfront, protected by seawalls too high for ramps to be dropped at any stage of the tide. Once past these barriers, the troops would have about two hours of daylight in which to secure an Oriental city with a population comparable to that of Omaha. But the amphibious assault was only the first phase of the operation as conceived by CinCFE. After taking Inchon the landing force had the task of driving some 16 miles inland, without loss of momentum, to assault Korea’s largest airfield before crossing a tidal river to assault Korea’s largest city.