Abstract
Abstract. To determine possible controls on sediment grain properties, 21 coastal rivers located along the entire western Peruvian margin were analysed. This represents one of the largest grain size dataset that has been collected over a large area. Modern gravel beds were sampled along a north–south transect on the western side of the Peruvian Andes where the rivers cross the tip of the mountain range, and at each site the long a axis and the intermediate b axis of about 500 pebbles were measured. Morphometric properties of each drainage basin, sediment and water discharge, together with flow shear stresses, were determined and compared against measured grain properties. Pebble size data show that the values for the D50 are nearly constant and range between 2 and 3 cm, while the values of the D96 range between 6 and 12 cm. The ratios between the intermediate and the long axis range from 0.67 to 0.74. Linear correlations between all grain size percentiles and water shear stresses, mean basin denudation rates, mean basin slopes and basin sizes are small to non-existent. However, exceptionally large D50 values of 4–6 cm were measured for basins situated between 11–12 and 16–17° S latitude where hillslope gradients are steeper than on average or where mean annual stream flows exceed the average values of the western Peruvian streams by a factor of 2. We suggest that the generally uniform grain size pattern has been perturbed where either mean basin slopes or water fluxes exceed threshold conditions.
Highlights
We suggest that the generally uniform grain size pattern has been perturbed where either mean basin slopes or water fluxes exceed threshold conditions
It has been proposed that the grain size distribution, of mountainous rivers, mainly depends on (i) tectonic uplift resulting in steepening of the entire landscape (Dadson et al, 2003; Wittmann et al, 2007; Ouimet et al, 2009), (ii) earthquakes and seismicity causing the release of large volumes of landslides (Dadson et al, 2003; McPhillips et al, 2014), (iii) precipitation rates and patterns controlling river discharge and shear stresses (D’Arcy et al, 2017; Litty et al, 2017), and (iv) bedrock lithology in which low erodibility lithologies are sources of larger volumes of material (Korup and Schlunegger, 2009; Allen et al, 2015)
If enough material is supplied to the streams, the grain size pattern mainly depends on the run-off and related shear stresses in these rivers, which in turn correspond to transport-limited conditions
Summary
The size and shape of gravels bear crucial information about (i) the transport dynamics of mountain rivers (Hjulström, 1935; Shields, 1936; Blissenbach, 1952; Koiter et al, 2013; Whittaker et al, 2007; Duller et al, 2012; Attal et al, 2015), (ii) the mechanisms of sediment supply and provenance (Parker, 1991; Paola et al, 1992a, b; Attal and Lavé, 2006), and (iii) environmental conditions such as uplift and precipitation (Heller and Paola, 1992; Robinson and Slingerland, 1998; Foreman et al, 2012; Allen et al, 2013; Foreman, 2014). McLaren and Bowles, 1985; Lisle et al, 1993) and numerical models (Hoey and Ferguson, 1994) These studies have mainly been directed towards exploring the controls on the downstream reduction in grain size of gravel beds (Schumm and Stevens, 1973; Hoey and Fergusson, 1994; Surian, 2002; Fedele and Paola, 2007; Allen et al, 2016). If enough material is supplied to the streams, the grain size pattern mainly depends on the run-off and related shear stresses in these rivers, which in turn correspond to transport-limited conditions
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