Boundary conditions for damming of a large river by fallout during the 12,900 BP Plinian Laacher See Eruption (Germany) Syn eruptive Rhine damming II
Year: 2020

The Rhine River (Germany) the largest river in Western Europe was dammed by pyroclastic material multiple times during the major Plinian Laacher See Eruption (12,900 BP) Dams formed both upstream and downstream of the broad tectonic Lower Neuwied Basin (LNB) which interrupts the narrow Rhine canyon. Here we document upstream damming of the Rhine River at the entrance to the LNB close to the present city of Koblenz due to overloading with tephra fall into the Rhine and its major tributaries, the Moselle and the Lahn. The dam was formed repeatedly during rapid pumiceous tephra fall events and breached during breaks in eruptive activity, causing extensive, high energy flooding throughout the entire basin. The ephemeral Koblenz dams differed significantly from "normal" volcanically induced dams by consisting principally of washed together pumice clasts and some driftwood. The porous nature of pumice and its ability to absorb water were crucial factors. Thus, a large volume percentage of the tephra that had fallen into the Rhine floated submerged within the upper part of the water column or swam at the surface. Moreover, the absorption of the river water by the pumice clasts increased the sediment:water ratio of the two phase flow considerably. We here present a model of dam formation resembling the formation of ice jams. We visualize the Koblenz dams to have been elongate, partly floating and partly grounded, permeable plugs many kilometers long and rising no higher than the flood plain. Damming was most plausibly initiated in the LNB within the area of maximum tephra loading and propagated upstream in a chain reaction comparable to the formation of traffic jams. A major dam was finally accumulated at the bottleneck entrance to the LNB, a site combining several favorable conditions: the upstream multi channel Rhine was confined to a single channel, change of flow direction by 125deg, extremely low gradient (0.19%0) starting already 24 km upstream of the bottleneck, constant decrease of flow velocity over many kilometers towards the bottleneck and the Moselle River largest tributary of the Rhine within the LNB and an important conveyor of additional tephra masses entered the Rhine only 700 m upstream of the bottleneck. We assume that the Koblenz dams could only have formed and been stabilized by an extremely long "foot region" that extended many kilometers downstream and that was possibly connected to one or several low rise secondary jams/dams. The backwater of Lake Brohl that was dammed by pyroclastic flows 7 km downstream of the LNB about halfway through the eruption extended further and further upstream into the LNB during the second Plinian stage of the Laacher See Eruption and was probably a major factor contributing to the formation and large size of Koblenz Dam No.4. The Koblenz dams were probably not completely sealed most of the time. This way the major pre eruptive Rhine channel received some water. An equilibrium condition was established that enabled the dams to remain stable as long as tephra fell into the Rhine relatively continuously.