A few weeks ago, we went down to the lab and played with the flume to simulate a hyperpycnal flow. This was cool on its own to watch, but I was struck by the how some of the water we poured in sat in one of the basins. When the water was originally added, it applied a lot of shear stress to the sediment in the flume, and so created the large scour on the right side. Then, most of the water flowed outward to the rest of the flume, but some stayed in the mini basin we’d created. As seen in the picture above, the darker blue water, which constituted the hyperpycnal flow, stayed very neatly in this basin. I found it surprising that, although the only thing above the denser water was other water, it was still incapable of escaping the basin, and was held there just like sediment would be. Eventually, though, the cooler, more saline water would have begun to mix with the ambient fluid, and we would see the water leave the basin.