Not a stream. Here we are looking at allochthonous input onto the edge of a lava flow, from the forest beyond. On this young lava flow (in the Oregon Cascades), I found substantially greater soil depth near the edge of the flow, where organic acids from decaying allochthonous organic matter had probably sped up the weathering process, as well as contributing directly to the soil. In my PhD dissertation, one subsection had “allochthonous inputs” for a title.