Sentence:  The angle at which the <WSD>waterbar</WSD> lies across a trail is important. Too close to the perpendicular, and silt will build up behind the <WSD>waterbar</WSD>; too small an angle will cause the trail to wash out. 
Meaning of the word marked with <WSD> tags:  A barrier built across a steeply sloping road or trail in order to direct the flow of water and prevent erosion; waterbreak.