Thus emplaced, the shape of a lava dam resembles an elongated blob, wedged in the valley bottom. At the same time, the water continues to flow, the lake continues to rise and accumulate sediment, which previously had migrated unimpeded downstream. Sediment filling, over-topping, downward erosion, waterfalls and under-cutting inevitably follow,[1] unless an alternative outlet is established, for water and sediment elsewhere in the drainage.