The dense mixture of sediment and water flowed almost vertically down to the bottom of the tank. It then flowed along the bottom, mixing somewhat with the clear water, until it reached the low end of the tank. Then, if it still had enough energy of motion, it turned and flowed at least part way back up the slope towards the high end of the tank. The students have made a turbidity current. Their turbidity current behaved very much like turbidity currents that form in the ocean.
Turbidity currents are important to people because they damage equipment on the ocean floor.
Also, turbidity currents are one of the most powerful mechanisms that move sediment to the deep ocean. If they performed the optional second part of the experiment in which they poured hot water into cold water, then they made a density current created by temperature differences. These are not common in nature, but they are similar to density currents formed by differences in the amount of salt dissolved in water. Salt makes water denser, so the less-dense fresh water flows on top of the denser sea water. I suggest that the students use a temperature difference to create a density difference in order to show that density currents can be created by completely different mechanisms.