What made the run to dyea cañon do difficult?

"It was a hard day's run, up the Canon, through Sheep Camp, past the Scales and the timber line, across glaciers and snowdrifts hundreds of feet deep, and over the great Chilcoot Divide, which stands between the salt water and the fresh and guards forbiddingly the sad and lonely North. They made good time down the chain of lakes which fills the craters of extinct volcanoes, and late that night pulled into the huge camp at the head of Lake Bennett, where thousands of goldseekers were building boats against the break-up of the ice in the spring. Buck made his hole in the snow and slept the sleep of the exhausted just, but all too early was routed out in the cold darkness and harnessed with his mates to the sled.

That day they made forty miles, the trail being packed; but the next day, and for many days to follow, they broke their own trail, worked harder, and made poorer time. As a rule, Perrault traveled ahead of the team, packing the snow with webbed shoes to make it easier for them. Francois, guiding the sled at the gee- pole, sometimes exchanged places with him, but not often. Perrault was in a hurry, and he prided himself on his knowledge of ice, which knowledge was indispensable, for the fall ice was very thin, and where there was swift water, there was no ice at all.

Day after day, for days unending, Buck toiled in the traces. Always, they broke camp in the dark, and the first gray of dawn found them hitting the trail with fresh miles reeled off behind them. And always they pitched camp after dark, eating their bit of fish, and crawling to sleep into the snow. Buck was ravenous. The pound and a half of sun-dried salmon, which was his ration for each day, seemed to go nowhere. He never had enough, and suffered from perpetual hunger pangs. Yet the other dogs, because they weighed less and were born to the life, received a pound only of the fish and managed to keep in good condition."