Take this for what its worth, but you really need to get someone to take a serious editorial look before you put any effort into trying to get that published. The concept could work, but that excerpt has problems in abundance. For example the first paragraph:
The creek was thirty feet across here. Depth was indeterminate toward the unsurveyable middle, but for ten feet off the bank the clay bed could be easily seen through no more than three feet of water
The first three sentances focus on detailed measurements, but fail to really give any detail about the surroundings other than the river has a clay bed. I call this "Kevin J. Anderson standard time part syndrome" The exact width is limiting. A horse is at least 6 feet long, and can jump at least 12 feet from a slow walk. assume that there is at least 10 feet of creek that is shallow on either side, that doesn't leave alot of room to almost drown in. We don't need to know how shallow the section next to the banks are, simply noteing they are shallow is enough. That space should be used to build a picture of the scene. Something along the lines of "The creek was narrow, with broad shallow clay banks. Easy enough footing for tired horses on the edge, but swirling clay silt and the play of the afternoon shadows from the trees along the bank made the depth at the center impossible to judge, even for the practiced eye of a U.S. Marshal used to such crossings."
The water’s surface was unblemished, and they saw no debris to dodge save the swirling red sediment stirred up with every step. The Lawman coaxed his horse in, and Junior followed
The second section is somewhat better, it paints a picture of the water flow of the channel, as a deceptivly smooth looking body of water. The main problem is the sediment being stirred up is mentioned before any of the horses actually get in the water.
Those are just some quick examples from the first part of the excerpt. All of the rest of it contains similar issues. I'm not saying this to be an ass, but it needs someone with a good editorial eye to go over it before you try to get it published if you want to make a real run at it.