the tolerances were fine , the plan had to be nicely judged ; besides falling just the right amount , at the right gradient , the channels had to be planned to follow the contours , to clear every stagnant pond and marshy dip ; and that was not so easy , the geometry of the meandering channels would turn back on itself if you were n't careful , or you 'd end up leaving a morass in some crease of the landscape undrained .