Controls on modern tributary-junction alluvial fan occurrence and morphology: High Atlas Mountains, Morocco

Abstract

Modern tributary-junction alluvial fans (cone-shaped depositional landforms formed in confined valley settings) were analysed from a 20-km-long reach of the Dades River in the distal part of the fold-thrust belt region in the south-central High Atlas Mountains of Morocco. Here, a deeply dissected network of ephemeral tributary streams and a perennial trunk drainage characterised by an arid mountain desert climate are configured onto a folded and thrust faulted Mesozoic sedimentary sequence. Out of 186 tributary streams, only 29 (16%) generated alluvial fans at their tributary junctions. The fan-generating catchments possess higher relief, longer lengths, lower gradients, and larger areas than nonfan-generating catchments. Whilst geologically, fan-generating catchments are underlain by folded / steeply dipping weak bedrock conducive to high sediment yield. Tributary-junction fans are built from debris flow or fluvial processes into open or confined canyon trunk valley settings. The proximity of the perennial trunk drainage combined with the valley morphology produces lobate or foreshortened trimmed fan forms. Analysis of fan (area, gradient, process), catchment (area, relief, length, gradient), and tributary valley (width) variables reveals weak morphometric relationships, highlighted by A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 2 residual plots that show dominance of smaller and lower gradient than expected fan forms. These morphometric relationships can be explained by interplay between the catchment and trunk drainage geology, morphology, climate, and flood regime that are combined into a conceptual 'build and reset' model. Ephemeral tributary-junction fans develop progressively during annual localised winter-spring storm events, attempting to build toward a morphological equilibrium. However, the fans never reach an equilibrium morphological form as they are reset by rare (>10 year) large floods along the River Dades that are linked to regional incursions of Atlantic low pressure troughs. The model highlights the spatial and temporal variability of tributary-junction fan building and illustrates the connectivity / coupling importance of such features in dryland mountainous terrains.

Tributary‐junction fans as buffers in the sediment cascade: a multi‐decadal study

“…Al-Farraj and Harvey, 2005;Crosta and Frattini, 2004;Stokes and Mather, 2015). Surprisingly, fan area was only weakly related to basin area ( Figure 11E), in contrast to numerous other studies (Al-Farraj and Harvey, 2005;Kovanen and Slaymaker, 2008;de Scally and Owens, 2004;Stokes and Mather, 2015). Fan area was more strongly related to the ratio of tributary to mainstem basin area ( Figure 11F), implying that the discharge ratio controls fan growth and persistence.…”

“…Such a threshold was identified by Wang et al (2008) and Stokes and Mather (2015), and attributed to within-basin sediment storage in larger tributaries. This implies that there is a basin area threshold, above which fan persistence is less likely.…”

“…The latter can limit tributary fan progradation (Al-Farraj and Harvey, 2005;Crosta and Frattini, 2004;Stokes and Mather, 2015). The latter can limit tributary fan progradation (Al-Farraj and Harvey, 2005;Crosta and Frattini, 2004;Stokes and Mather, 2015).…”

“…Al-Farraj andHarvey, 2005, 2010;Gómez-Villar et al, 2006;Mather and Stokes, 2018;Stokes and Mather, 2015;Wang et al, 2008). Al-Farraj andHarvey, 2005, 2010;Gómez-Villar et al, 2006;Mather and Stokes, 2018;Stokes and Mather, 2015;Wang et al, 2008).…”

“…Typically, these studies have used 'space for time' substitution, with a large sample (n > 60) used to investigate how fans differ as their controlling forces vary across space (e.g.Al-Farraj and Harvey, 2005;Crosta and Frattini, 2004;Stokes and Mather, 2015). Typically, these studies have used 'space for time' substitution, with a large sample (n > 60) used to investigate how fans differ as their controlling forces vary across space (e.g.Al-Farraj and Harvey, 2005;Crosta and Frattini, 2004;Stokes and Mather, 2015).…”