Tufa Runnel is not a scientific term. Unfortunately we could not find the english term for this rather common and impressive structure, it seems it has not been described in english literature so far. So this is a try to find a descriptive term.
A tufa runnel is a typical formation of tufa or travertine, close to karst springs. The water from the karst spring is very rich in dissolved limestone, which is only possible because of its high content of carbon dioxide. But as soon as the water reaches the surface, the carbon dioxide leaves the water into the air and the limestone is deposited. Commonly if forms dams, lakes, and huge deposits of tufa.
But in some cases, it the water flows down a certain slope, immediately after the spring, the water is too fast to form dams. The tufa is deposited primarly as shoreline tufa along the brook, and also as stream-bed tufa. The result are two tufa dams along the water, which grow continually forming a dam. At the same time limestone depositition in the stream bed lifts the brook, and so after some time the brook has forme a sort of wall, where it rans on top.