Ignore the end of the post. Here I was making a brilliant answer to hove the mount couldn't cross that dretch easily, and after posting, I notice that the knight was
unmounted.

Oh well, not going to remove all that nice analyzation just for that, maybe somebody will get good laughs off my stupidness from it.
I did, however, notice one interesting thing. In d20 Modern, heavy armor does not decrease one's running speed to x3 according to the SRD (and 55ft would still be just within the character's running distance in DnD too).
((What's the average weight of a human male in d20 Modern? I'm working off the SRD, and with a quick look-through, I couldn't find that information. However, the weight that the horse is carrying is 75lbs from the equipment that the character has, plus some lbs for a saddle, plus the weight of the character. And the horse's light load limit is 150lbs.
So unless the knight is really thin (weighting in at only around 70lbs), the horse has a -8 (+1 Str, -6 speed 20ft, -3 Medium load) Jump modifier (and when jumping with a mount, you use the rider's Ride or the mount's Jump, whichever is less), so making that Jump check to cross the drench isn't such an easy thing after all unless the drench less than 5ft in width. For a 5ft drench, the mount actually needs to jump 10ft to get its whole area over the drench (otherwise its rear would end up in the drench), needing to roll an 18 on the Jump check. So the soldier likely has several rounds before the knight gets over that drench.))