Four alluvial fans, Pulanera, L'Ondadusa, Drun and Bugnei, located near the town of Sedrun in the upper Vorderrhein Valley (Fig. 1), are striking in that they have a common aggradation surface (∼ 1∘ slope oriented along the main valley axis) and distinct coterminous cut-off toes. Similar coalesced fans with marked incised toes occur just downstream at Disentis (Fig. 2a). These geomorphological characteristics suggest that the fans built up for a limited period of time and then were abruptly incised by the Vorderrhein River. Switching between aggradation, fan abandonment and channel entrenchment can be a response to (1) variation in discharge due to climate changes or extreme precipitation events, (2) decreases in sediment supply reflecting waning (e.g. paraglacial) sediment availability, (3) tectonic uplift, or (4) changes in base level (Harvey et al., 2005; Hornung et al., 2010, and references therein). Inside a mountain range, change in local base level can be caused by large-scale downstream blockage by a glacier, a landslide or a man-made dam (Korup and Tweed, 2007).

Previous work in the study area was focused on the bedrock (Huber, 1948), especially for characterization of the NEAT Gotthard basis tunnel site below Sedrun (Schneider, 1992, and references therein). In the northern tributary valleys (Val Giuv, Val Mila, Val Strem; Fig. 2a) Aar granite dominates, while strongly sheared metamorphic rocks of the Tavetscher Zwischenmassif underlie the Vorderrhein Valley itself. Along the trough shoulder, especially at Cuolm da Vi (Fig. 2a), active deep-seated slope degradation is present as evidenced by abundant trenches and uphill facing scarps (Amann et al., 2006).

The aim of this study is to determine what caused this abrupt and apparently brief period of fan aggradation in the upper Vorderrhein. Previous researchers have interpreted the Sedrun and Disentis fans as kame terraces (Scapozza, 2012) or as Lateglacial outwash fans (Hantke, 1983). For the latter to be true the valley fill at Sedrun would have to be dominated by tributary valley sediment rather than Vorderrhein fluvial sediments. Both of these hypotheses require that the fans formed between about 17 and 11.7 kyr ago.

To decipher the evolution of the upper Vorderrhein region and the origin of the Sedrun fans, we did detailed geomorphologic mapping supported by ArcGIS landscape analysis, as well as cosmogenic 10Be exposure dating of boulders on moraines at the mouths of the three tributary valleys. No boulders suitable for exposure dating were found on any of the fan surfaces. Debris-flow sediments of the fans bury several of these moraines; thus the moraine dates provide maximum ages for aggradation of the fans. Sampling, sample preparation and age calculations for 10Be exposure dating followed established procedures (Ivy-Ochs and Kober, 2008). Based on the mapped lateral and frontal moraines, paleoglaciers were reconstructed and the associated equilibrium line altitudes (ELAs) were calculated using the ArcGIS GlaRe tool (Pellitero et al., 2015, 2016). Knowledge of the ELA depression with respect to the Little Ice Age ELA is another means of estimating the timing of the glacier advance and associated moraine construction. We reconstructed the sedimentary fill of the upper Vorderrhein Valley based on our own outcrop descriptions combined with the information gleaned from the published NEAT core logs (Schneider, 1992).

The dated left-lateral moraine at the mouth of Val Mila (Mila2) is buried along its downstream end by fan sediments sourced in the small basin Val Pulanera. In Val Strem three left-lateral moraines are partially buried by the Drun upper fan deposits (Fig. 2c). Although those moraines are undated, the similarity of the paleoglacier ELAs and ΔELAs with respect to Little Ice Age ELAs point to contemporaneity of moraine formation with the dated moraines in Val Giuv (12.9–10.9 kyr ago). As the footprint of the paleoglacier snout associated with the dated moraines (Fig. 4) was downstream of the Drun–Strem confluence, debris-flow activity post-dates the Egesen stadial. Similarly, the core data show that only the top 35–40 m of valley fill is actually debris-flow sediments; most is fluvial and glaciofluvial deposits of the Vorderrhein River, which accumulated in the overdeepening upstream of Bugnei. In sum, our data indicate that fan sediments built up after the end of the Lateglacial and that aggradation only took place for a few tens to at most hundreds of years (deposit thickness 30–40 m), when one considers that tens of metres of debris-flow sediment can accumulate in a single event (Harvey et al., 2005).

If the fans did not build up due to evacuation of Lateglacial glacial deposits from the tributary valleys, then why did they aggrade and why did aggradation stop and give way to toe incision so abruptly? We propose that the build-up of the Sedrun and Disentis fans was related to the damming of the Vorderrhein River by the Flims landslide. The Flims landslide (volume 9–12 km3) has been dated to 9400 cal yr BP with radiocarbon (Deplazes et al., 2007) and 8900 ± 700 years with cosmogenic 10Be and 36Cl (Ivy-Ochs et al., 2009). The Vorderrhein was completely blocked by a more than 600 m-thick landslide dam and a lake, Ilanzersee, formed upstream (Wassmer et al., 2004; von Poschinger, 2005). This would have led to marked upstream aggradation for as long as the lake existed. Interestingly, in 1882 Heim mentioned that the Flims landslide-dammed lake had an impact on the landscape all the way up to Disentis, without giving details (Heim, 1882). The highest elevation of the lake is difficult to assess, especially as it may not have been at that elevation for very long. Wassmer et al. (2004) suggest a lake level of 1150 m a.s.l. but geomorphological evidence shows that a lake that high would have overflowed the landslide at numerous locations (cf. von Poschinger, 2005). It seems that the lake could have attained a level of around 940 m a.s.l. without overflowing (Fig. 5). Von Poschinger (2005) estimated a highest level of 936 m a.s.l. and a level that was held for fairly long at about 830 m a.s.l. As soon as the lake had filled with water (taking many years, disregarding seepage), a new higher local base level for all torrents upstream of the lake was established. The marked flattening of the river gradient and reduction in stream power led to immediate and rapid aggradation upstream. First to aggrade were the fans near Disentis whose fan toes were closest to the western end of the lake. At present, there is no evidence that the Disentis fans were deltas. As these fans grew they blocked the Cavorgia canyon (Fig. 2). Vorderrhein sediment choked the canyon and the Bugnei fan began to build up. This led to formation of accommodation space around Sedrun. As shown by the core data, first a lake formed west of Bugnei (recorded by sand layers between 1330 and 1340 m a.s.l. in core SBB), then the three fans, Pulanera, L'Ondadusa and Drun, aggraded and coalesced to form a continuous surface with an elevation of about 1380 m a.s.l. Ilanzersee never filled completely with sediment but emptied (likely stepwise) due to breaches in the landslide deposit where the present Vorderrhein channel is. The uniformity of the marked cut-off toes of the Disentis and Sedrun fans suggests coherent response to an abrupt downstream change in base level, which we suggest was the draining of Ilanzersee. As the lake level decreased suddenly, the Vorderrhein was able to clear out the Cavorgia canyon (there is no evidence for formation of a new bedrock canyon epigenetically), base level dropped and fan aggradation ceased. The fan toes were incised. The associated knickpoint is now located near the outlet of the Drun torrent just west of the Bugnei fan, as attested by the bedrock outcrops in the channel at elevation 1340 m a.s.l. This is also the approximate elevation of the present floodplain upstream of this point. The broad floodplain (200 m wide) and the nine cut terraces reflect meandering of the Vorderrhein near Pulanera and point to the river being in a stable state at present along this reach.