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

Abstract

Alluvial fans at tributary junctions modulate sediment flux through river networks, by buffering the mainstem channel from disturbance in the tributaries. Buffering occurs through the storage (and release) of sediment in fans. Here, we use an extensive historic dataset to characterise the ways in which fan buffering can change as sediment supply varies. In New Zealand's East Coast region, sediment supply and fluvial transport are prolific by global standards. We reconstruct how tributary‐junction fans in this region have responded to sediment generated by deforestation and extreme storms. The dynamics of five fans along the Tapuaeroa River are examined for the period 1939–2015. In response to major sediment loading, fans aggraded by up to 12 m and prograded by up to 170 m. Net sediment accumulation ranged from near zero to 1.5×106 m3. Fan size, gradient, sediment storage and buffering were influenced by both upstream and downstream controls. Key upstream (tributary) influences were sediment supply and stream power; downstream (mainstem) influences included distal confinement and, importantly, the nature of fan interaction with the mainstem, which aggraded by up to 6 m. The fans' ability to buffer the Tapuaeroa River from change in the tributaries was largely governed by this downstream interaction: as the mainstem aggraded, it increasingly curtailed fan progradation, thus limiting buffering. Previous studies of tributary‐junction fans have related fan morphometry to basin characteristics. However, we find that fan slope and area can vary considerably at decadal, annual or even monthly timescales. Consequently, we suggest that such studies could benefit by examining regional histories of disturbance. © 2019 John Wiley & Sons, Ltd.

Evolution of tributary junctions and their capacity for rearing juvenile Chinook salmon (Oncorhynchus tshawytscha) on a regulated river

“…The establishment and merging of vegetation patches also occurred at Indian Creek, but the braided fan instead evolved to a narrow, single thread channel despite the relatively high sediment loads on the creek and mainstem river. These changes resulted from the narrowness of the valley providing little space for the vegetationconstricted fan to enlarge, which supports previous assertions that valley width exerts a first-order control on fan size (Leenman & Tunnicliffe, 2020;Stokes & Mather, 2015). We also posit that valley width can modify vegetation effects from promoting fan aggradation and areal expansion in wide valleys to limiting fan growth and persistence in narrow valleys subjected to regulated mainstem flow regimes.…”

“…The dynamics result from abrupt changes in streamflow, sediment supply and grain size by tributary inputs that can alter pathways for flow and sediment transport through the junction (Best, 1988;Imhoff & Wilcox, 2016;Rice et al, 2006). These inputs often result in mainstem adjustments that can include local increases in sediment calibre and downstream channel slope (Rice 1998;Benda et al, 2003), alluvial fan and bar aggradation and erosion (Leenman & Tunnicliffe, 2020), bank retreat and floodplain formation varying channel width and alternating destruction and creation of aquatic habitats, including shear zones, low velocity areas and alcoves.…”

“…The dynamics result from abrupt changes in streamflow, sediment supply and grain size by tributary inputs that can alter pathways for flow and sediment transport through the junction (Best, 1988; Imhoff & Wilcox, 2016; Rice et al, 2006). These inputs often result in mainstem adjustments that can include local increases in sediment calibre and downstream channel slope (Rice 1998; Benda et al, 2003), alluvial fan and bar aggradation and erosion (Leenman & Tunnicliffe, 2020), bank retreat and floodplain formation varying channel width and alternating destruction and creation of aquatic habitats, including shear zones, low velocity areas and alcoves. Habitat complexity and delivery of nutrients and organisms by tributaries have led many to label tributary junctions as biological hotspots (Benda et al, 2004; Polis et al, 1997; Power & Dietrich, 2002) that disproportionately contribute to the aquatic productivity, biodiversity and physical complexity in a river network (Kiffney et al, 2006; Rice et al, 2008).…”

Episodic sediment supply to alluvial fans: implications for fan incision and morphometry

“…Following the 2010 landslide and debris flow at Mount Meager, British Columbia, debris-flow sediment completely filled a creek in an incised trench of 5-15 m depth (a ratio of 1.0), thinning downstream to around 2 m (a ratio of 0.1-0.4) (Guthrie et al, 2012). In northeastern New Zealand, post-logging deposition on fans ranged from < 1 to ∼ 10 m in depth, in one case filling an incised fan-head trench by 12 m (Leenman and Tunnicliffe, 2020). These comparisons demonstrate how the size of depositional events relative to fan-channel dimensions are similar in the model and some field examples.…”

“…de Haas et al, 2018a;Field, 2001;Zubrycky et al, 2021); sequences of multiple events may be required to affect whole-fan morphology, implying that these individual events are shorter than the adjustment timescale. Conversely, on fans experiencing longer (decadal) periods of heightened sediment supply, prolonged aggradation and progradation across the whole fan have been observed (De Rose et al, 1998;Leenman and Tunnicliffe, 2020;Marden et al, 2018). The balance between the magnitude, duration, and frequency of sediment inputs and the timescale of fan adjustment is therefore key for understanding how fans respond to episodic sediment inputs.…”