In Fahrenheit 451 where was Montag when the Mechanical Hound reached the river?
By the time Montag reaches the river, he has been on the run for quite a while. Montag strips his own clothes and throws them down the river, douses himself in alcohol to mask his own scent, then puts on Faber's clothes. He is hoping that all of these little tricks will distract the hound from the trail, and give Montag a bit if a chance to escape. After he dresses in Faber's clothes, he jumps into the river and floats downstream. He doesn't get much of a break though, because
"he was three hundred yards downstream when the Hound reached the river."
Montag panics and dives under the water, hoping that the helicoptor, that accompanies the hound wherever it goes, won't spy him floating along. He dives under and when he comes back up, it is much quieter--the hound and helicoptor have picked up a different trail and have gone a different direction. Montag thwarted them at last. He is relieved, and able to float along for a while before getting out on the opposite side to find somewhere to rest.
Montag will learn later that even though he escaped, the chase still continues; on the television screens, an innocent man is taken down, one that his government hopes people will believe is him. Montag's leap into the river barely ensured his escape, but unfortunately, an innocent victim succumbs to the hound anyway. I hope that those thoughts helped; good luck!