Confusing Shrub-steppe Microclimate

This dry shrub-steppe region sees only 8 or 9 inches of rain annually.  On the upper banks of the rail grade you can see sagebrush and rocky soil sustaining little else.

I was riding along and wondering about the presence of willow saplings -- trees that only grow in moist conditions.  Then I saw bulrushes, and my confusion was complete when my wheels were squelching into a boggy bed of watercress.

This moist and shady microclimate was created a century ago when cutting the grade for the railway uncovered a groundwater source.  It's now a serious obstacle to through hikers and cyclists on the trail.