Root wads are the root systems of upended trees. When the tree falls, its Medusa-like tangle appears with clumps of dirt and rock stuck among the separate roots, suspended now above a hole called a tip pit. Root wads that fall in wetlands or waterways such as streams and rivers are microhabitats for ﬁsh and aquatic invertebrates, which in turn provide food for ﬁsh, birds, and amphibians. The fallen tree also serves as a bridge between terrestrial and aquatic habitats, and young ﬁngerling salmon, hatched in inland rivers, have been known to ride within root wads all the way to the ocean. Anthropologists and archeologists scrutinize root wads for evidence of shell middens, bones, and tools. An Alaskan ornithologist once used root wads of trees that had drifted ashore around Barrow to provide nest sites for black guillemots. Root wads have also been used by bioengineers in stream restoration projects. The roots act as sieves for debris, and help clean the streams. They are also used along the banks to reduce erosion. Sometimes several root wads are combined and buried into a streambank with the root fans exposed to the current, thus slowing the stream, and slowing the erosion of cutbanks.