Buried and forgotten—The non‐fluvial characteristics of postglacial rivers

“The systematic analysis and understanding of channel‐forming processes of rivers must be expanded by including semi‐ and non‐fluvial geomorphological processes. Such processes were particularly driven by glaciation during the Pleistocene and led to diamictic non‐fluvial deposits in the post‐glacial valleys. In the Holocene, rivers either covered these deposits with fluvial sediments or incised into them and exposed the non‐fluvial deposits. These processes have strong and so far overlooked implications for the understanding of the genesis, morphology and sediment composition of many rivers – and thus for river utilization, ecology, restoration and management.”


| CONTENT
In this article, we argue that the analysis and understanding of channel-forming processes of rivers must be revised because semiand non-fluvial processes in postglacial environments have been overlooked. During the glaciation periods of the ice ages, glaciers have shaped the geomorphology of large areas around the world (Esmark, 1824, Ehlers, Gibbard, & Hughes, 2011, particularly along mountain ranges and in arctic and subarctic regions (Figure 1). These processes had decisive effects on valley and river formation. To understand river genesis in such regions, we differentiate the geomorphological process in two phases, a glacial (i) and a postglacial phase (ii).
The glacial phase (i): During the Pleistocene, enormous glaciers burst, scratched, and dug the terranes of the northern and southern Hemispheres, reshaping the surface (Ehlers et al., 2011), with local differences, however, e.g. between "cold-based glaciers" and "warmbased glaciers" moving across the landscape (cf. Hodson & Ferguson, 1999). In the Holocene, ice shields melted and glaciers retreated. Glacial tills, glaciofluvial, and colluvial deposits remained on the valley floors-mainly of diamictitic composition (Figure 2, Olsen, Fredin, & Olesen, 2013). The deposits originated from glacial and colluvial processes. They were not sorted, and they had a wide range of grain-size distribution. Partly, scoured bedrock was exposed. In addition, glaciofluvial deposits may have had sorted sediments within layers but consisted of varying sediment composition between the layers (Corner, Dalrymple, Leckie, & Tillman, 2006). Thus, the initial postglacial valley fills are considered mainly as semi-and non-fluvial sediments . They formed the initial stage for river genesis in the glacier-shaped valleys of the world.
The postglacial phase (ii): Below glaciers and after they were gone, rivers reshaped the diamictic sediments of the valleys by fluvial processes (Corner et al., 2006;Gilbert et al., 2017. Usually, discharge was significantly higher than today because large amounts of ice were melting. Partly, outburst floods occurred with high potential for erosion and sediment transport. Material was transported along mountain slopes by colluvial processes and further downstream by fluvial processes. In drainages with tectonic orogeny, sedimentary rocks, intense erosion, and high sediment yields, the young rivers became transport limited (iia). The fluvially transported sediments were deposited on top of the non-fluvial valley bottom sediments. In such drainages, the valleys were filled to a large extent.
The largest deposition happened along major global mountain ranges, such as the Alps, the Himalaya, and the Rocky Mountains, where fluvial sediments in valleys usually reach several hundred metres thickness (Molnar & England, 1990;Preusser, Reitner, & Schlüchter, 2010).
However, this did not happen in all parts of the world. Depending on the climate, tectonics, and bedrock conditions, weathering processes and thus sediment yields were limited, for example, on the metamorphic and plutonic terrane of the Scandinavian shield without recent tectonic orogeny. In such areas, initial semi-and non-fluvial deposits from the glaciation phase remained uncovered and exposed on the valley floor. Rivers often became supply limited (iib) and incised into these deposits developing a characteristic of heterogeneous highly diverse morphology with fluvial, semi-, and non-fluvial reaches ).
Today's understanding of river genesis and river morphology classifications, however, mainly assume an entirely fluvially formed river environment (despite in confined reaches). Apart from that, they do not include non-fluvial sediments explicitly from a process-based point of view (Chin, 1998;Grant, Swanson, & Wolman, 1990;Hauer, 2015;Lisle, 1986;Montgomery & Buffington, 1997;Peterson & Mohanty, 1960;Phillips, 2002;Schumm, 1977;Wohl, 2013;Wohl & Merritt, 2008). State-of-the art analyses of river morphology can be descriptive, leading to the classification and differentiation of various channel patterns, or based on a systematic analysis of channelforming processes (Kasprak et al., 2016). However, the traditional methods in river research have one thing in common; the fluvial channel configuration is based on an equilibrium status (described as Lane's law, Lane, 1955). Stream channels are considered in equilibrium when the sediment discharge (Qs) * sediment particle size (D 50 )s treamflow (Q w ) * stream slope (S).
However, Lane's law (1955), which has become a fundamental principle in river science, is not valid in rivers systems dominated by semi-and non-fluvial sediments. Rivers running through a nonfluvially formed environment have incised mainly due to palaeohydraulic flood events (Q w ), including extraordinarily high flows due to the rapid meltdown of glaciers (Fairbanks, 1989; Figure 2).
During these incision processes, sediments are washed out according to the event-based fluvial tractive forces that lead to the pavement formation of very coarse non-fluvial sediments on the river surface ( Figure 2). Another geomorphological process leading to semi-and non-fluvial characteristics in rivers are colluvial rockfall and avalanches, which may not only be distributed in headwater regions but also along lower river sections .
Thus, the initial distribution of sediment particles that have not been fluvially transported is decisive for such rivers' genesis and morphology. Morphology and sediment composition do not reflect the event-based stream power (streamflow times the stream slope).
Lane's law (1955) is therefore not valid (Figure 2b). In such rivers, neither palaeo-hydraulic approaches are applicable that claim that the largest grain sizes are in line with the hydraulic forces of the largest flood event (Church, 1978). Only if sediment supply (Q s ) was higher than transport capacity in the Holocene (depending on the weathering processes, tectonics, and discharge), the non-fluvial patterns were covered by fluvial sediments (iib). In these rivers, the fluvial concepts of river genesis such as Lane's law can be applied. Admittedly, such rivers dominate in the world because sedimentary terrain and tectonic orogeny prevail (Figure 1). Nevertheless, non-fluvial deposits can be found in their valleys, buried under fluvial sediments (Preusser et al., 2010). Yet there are also many rivers still exposing semi-and nonfluvial deposits. Such rivers are described for Norway   fluvial reaches, such as pool-riffle types, are more exposed to discharge-triggered dynamics . Such implications have to be considered in a revised river classification system. Moreover, sediment composition is an elementary habitat feature for aquatic organisms in rivers Pulg, Vollset, & Lennox, 2019;Vannote, Minshall, Cummins, Sedell, & Cushing, 1980). Semi-and non-fluvial river reaches provide characteristic highly diverse sediment compositions that consist of a grain size distribution F I G U R E 2 Schematic illustration of the theory on the genesis of rivers in postglacial environments; top: initial stage (i). In the middle: the initial stage of diamictic deposits (yellow coloured) is buried by partially hundreds of metres of fluvially transported sediments (green coloured) (iia), incision of the river into the diamictic deposits after glacier retreat (yellow coloured deposit); (iib) [Colour figure can be viewed at wileyonlinelibrary.com] with a large range and large maximum grain sizes and high shelter availability for fish and invertebrates. Therefore, they are likely to have a strong impact on biological diversity and production, habitat quality, connectivity, and dispersal of fish, invertebrates, and (due to their stability) algae and macrophyte growth. The ecological implications underline the need to adapt existing channel classification approaches to include semi-and non-fluvial characteristics of rivers. and Development, as well as the Norwegian Water Resorces and Management Directorate (NVE, project: "Flaum og vassdragsmiljø i eit endra klima") are gratefully acknowledged. Moreover, we are thankful for the comments and suggestions for improvement of the manuscript by two anonymous reviewers.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.