The river Umgaril is one of the few waterways that begins in the Underdark and ends on the surface. It begins one tier under Imgangreth. Half-way into the Lith-Crillion Era, Sathubas, a region of Brucrumus's Tier 2, saw its water tables rising. The aboleth were behind all this, upsetting the water balance to create more living space. At the time, Ophudehk was projected to reach its limit in another five hundred years. The long-lived aboleth, attached to the the city, began the work of re-channeling water from above. They were careful in the process, making sure not to redirect too much into the lower depths. Nearing the end of the Lith-Crillion Era, the great lake around Ophudehk had become a sea, and many caves and tunnels were now underwater. Seeing the waters rise ever so slowly, many packed up and left. The holdouts, now close to water, became easy pickings for the aboleth. The grimlock city-state Akalgas worked long to counter the oncoming waters. Aided by earth elementals, they built a series of great water wheels and canals called the Akalgas Wheel. It is a marvel of engineering and construction that would impress even the people of Regulus. It took three centuries to finish this work. When it was done, it begin to lift water upwards for thousands of feet becoming the headwaters of the Umgaril river.
After paralleling the Sidari-Nyx, this slow-moving river passes through the surface city Valangrim. Beyond the city's walls, it flows south into the Droonesh swamp. From Valangrim, barges use this river to carry trade between the surface and the Underdark.