Multistage damming of the Rhine River by tephra fallout during the 12,900 BP Plinian Laacher See Eruption (Germany). Syn-eruptive Rhine damming I

Abstract

The Rhine - the largest river in Western Europe – was dammed during the Plinian Laacher See Eruption (LSE; 12,900 BP). Damming during the climactic Plinian episode of LSE occurred both upstream and downstream of the broad tectonic Lower Neuwied Basin (LNB) that interrupts the narrow Rhine canyon. We here document details of the upstream damming at the bottleneck entrance to the LNB near the present city of Koblenz. Our reconstruction is based on a high-resolution analysis and correlation of the complex intercalation of primary fallout tephra relics with fluvially reworked Laacher See Tephra in the LNB. Tephra units representing complete eruptive cycles repeatedly fell on drained ground in between one minor and 4major flooding events - even at the base of side channels that had been active prior to the LSE and that had been flooded by a preceding flooding event. This demonstrates that flooding occurred generally during breaks and not during fallout events. The repeated formation and breach of a dam at the upstream entrance of the LNB (Koblenz Dam) consisting of fallout components and driftwood washed together convincingly explains the multiple repetition of the drainage of the channels in the LNB followed by large-magnitude flooding in rapid succession. The strongly pulsating nature of the LSE reflected in multiple interruptions of eruptive activity fundamentally controlled the damming and flooding dynamics. The Rhine became completely blocked during distinct fallout phases due to overloading with pumice that had fallen into the river and its major tributaries. The temporary dam collapsed during eruptive breaks. This is the first recorded example of tephra fallout damming a major watercourse. The extremely low gradient of the Rhine River allowed the repeated accumulation of large volumes of water in a long, multi-phase dammed-up lake (Lake Koblenz) that extended along the upstream course of the river for up to c. 30km despite the low height (<10m) of the dams. Each breach of Koblenz Dam caused extensive and wide-spread erosion and reworking of freshly deposited tephra throughout the entire LNB up to 7km perpendicular to the major axial Rhine channel. The floods deposited striking, large-scale upper flow regime structures interpreted as in-phase wave draping, antidunes and chute-and-pool structures consisting largely of gravel-sized tephra components. Primary tephra sheets - several meters thick - became detached by undercurrents above impermeable boundaries and floated potentially along the full length of both active and abandoned channels. Large tephra bodies with the dimensions of a garage were lifted off by the flood waves and transported downstream for at least tens of meters.Damming of a major river while a large Plinian eruption is in full progress represents an extraordinary challenge for hazard mitigation. This is especially pertinent for an area close to, and downwind from, the vent and therefore simultaneously affected by massive Plinian fall such as in the LNB.