  With a toppling rule which generates metastable sites, we explore the
properties of a gradient-driven sandpile that is minimally perturbed at one
boundary. In two dimensions we find that the transport of grains takes place
along deep valleys, generating a set of patterns as the system approaches the
stationary state. We use two versions of the toppling rule to analyze the time
behavior and the geometric properties of clusters of valleys, also discussing
the relation between this model and the general properties of models displaying
self-organized criticality.
