Brushing: if you don't notice, it's working
If there's any task as menial yet crucial as clearing water bars, it's brushing. It can take over 4 hours per mile. Fortunately, MTB Missoula has a number of volunteers with giant weedwackers at the ready, keeping those sight lines open and your knuckles less bloody.
In the last few years, we've brushed out:
Wallman Saddle East
Son of Sidewinder
Mo-Z (yes, the tumble mustard gets that thick)
Water Bars, again and again
Of course a reroute like Fenceline isn't always practical or necessary, and there are some fun trails (Dropout, for example) that just need a little help keeping the water off.
Water bars are short dips in the trail which move the natural path of water away from the tread. They come in all shapes and sizes, which is why we continue to work with land managers to try and make sure the ones that are installed are suitable for mountain biking. It's also a great reason to have mountain bikers themselves out there clearing them out.
The farther water runs along the trail, the more sediment it carries. That sediment comes from the trail surface in an erosive process that eventually turns the trail into a deep rut. The sediment collects and fills in the very drainage features designed to prevent this -- sometimes even one big storm can fill them, causing water to stay on the trail and hasten the erosion. So every year, sometimes twice, we send crews all over the Rattlesnake (and periodically Pattee Canyon) to scrub out the drains with shovels and keep the trail tread as erosion-free as possible.
And then there's Bill. Bill Schultz has probably cleared more water bars alone than the rest of us combined.