During the great western migration to California in the late 1840s and 1850s, neither excited gold-seekers nor families simply wanting to settle could depart overland for the Pacific Coast at just any time they wished. Theirs was a limited window of travel time, framed by potential hazards at both ends of the continent. Since their sole means of transportation was draft animals, they had to wait for spring grasses to sprout on the prairies to provide fodder for their wagon-hauling oxen and other livestock. Normally, this was the month of April or early May. Their back-end deadline was to safely cross the treacherous Sierra Nevada Mountain heights before the winter snowfall that might begin at the end of October. They had, in effect, six months or less to cross 2,000 miles from the Missouri River over largely unmapped or at best only vaguely mapped territory. It was imperative that their wagon trains moved along at a brisk pace, at least six days a week from sunrise to sunset (with a mandatory stop at lunch to rest and feed the animals), to allow for the inevitable slowing at river crossings or mountain obstacles. Average travel distance on flat plains was 14-20 miles a day, reduced to 3 or 4 miles (or less) per day through difficult terrain. Only very small children and the desperately sick rode inside the covered wagons. Adults and youngsters walked the trails to spare their animals from the burden of unnecessary weight. Few wagon trains braved the southern route via the Old Spanish Trail, because of the scarcity of water.