And since sugar produces thirst, and Silver had not smelled water since morning, he licked the last sweet grain from the inside of the nose bag and then walked out of the stall and the stable and made for the creek--and a horse cannot drink with a nose bag fastened over his face.

All he can do, if he succeeds in getting his nose into the water, is to drown himself most expeditiously and completely.
Silver reached the creek unseen, sought the deepest hole and tried to drink.

Since his nose was covered with the bag he could not do so but he fussed and splashed and thrust his head deeper until the water ran into the bag from the top.

He backed and snorted and strangled, and in a minute he fell.

Fortunately he struggled a little, and in doing so he slid backward down the bank so that his head was up the slope a and the water ran out of the bag, which was all that saved him.
He was a dead horse, to all appearances at least, when Slim spied him and gave a yell to bring every human being on the ranch at a run.