Climate‐driven alluvial fan aggradation and incision in an unglaciated Himalayan basin, Northwestern India

A three‐dimensional numerical model of sediment transport, erosion and deposition within a network of channel belts and associated floodplain is described. Sediment and water supply are defined at the upstream entry point, and base level is defined at the downstream edge of the model. Sediment and water are transported through a network of channels according to the diffusion equation, and each channel has a channel belt with a width that increases in time. The network of channels evolves as a result of channel bifurcation and abandonment (avulsion). The timing and location of channel bifurcation is controlled stochastically as a function of the cross‐valley slope of the floodplain adjacent to the channel belt relative to the down‐valley slope, and of annual flood discharge. A bifurcation develops into an avulsion when the discharge of one of the distributaries falls below a threshold value. The floodplain aggradation rate decreases with distance from the nearest active channel belt. Channel‐belt degradation results in floodplain incision. Extrinsic (extrabasinal, allogenic) and intrinsic (intrabasinal, autogenic) controls on floodplain dynamics and alluvial architecture were modelled, and sequence stratigraphy models were assessed. Input parameters were chosen based on data from the Rhine–Meuse delta. To examine how the model responds to extrinsic controls, the model was run under conditions of changing base level and increasing sediment supply. Rises and falls in base level and increases in sediment supply occurred over 10 000 years. Rising base level caused a wave of aggradation to move up‐valley, until aggradation occurred over the entire valley. Frequency of bifurcations and avulsions increased with rate of base‐level rise and aggradation rate. Channel‐belt width varied with water discharge and the lifespan of the channel belt. Wide, connected channel belts (and high channel‐deposit proportion) occurred around the upstream inflow point because of their high discharge and longevity. Less connected, smaller channel belts occurred further down‐valley. Such alluvial behaviour and architecture is also found in the Rhine–Meuse delta. During base‐level fall, valley erosion occurred, and the incised valley contained a single wide channel belt. During subsequent base‐level rise, a wave of aggradation moved up‐valley, filling the incised valley. Bifurcation and avulsion sites progressively moved upstream. Relatively thin, narrow channel belts bordered and cut into the valley fill. These results differ substantially from existing sequence stratigraphy models. The increase in sediment supply from upstream resulted in an alluvial fan. Most bifurcations and avulsions occurred at the fan apex (nodal avulsion), and channel belts were the widest and the thickest here (giving high channel‐deposit proportion) due to their high discharge and longevity. The width and thickness of channel belts decreased down‐valley due to decreased discharge, longevity and aggradation rate. This behaviour occurs in modern alluvial fans. Intrinsic controls also affect floodplain dynamics and alluvial architecture. Variation of aggradation rate, bifurcation frequency and number of coexisting channel belts occurred over periods of 500 to 2000 years, compared with 10 000 years for extrinsic controls. This variation is partly related to local aggradation and degradation of channel belts around bifurcation points. Channel belts were preferentially clustered near floodplain margins, because of low floodplain aggradation rate and topography there.

A basic tenet of arid region geomorphology is that a decrease in hillslope vegetation density coupled with increases in monsoonal precipitation after a change to a warmer and drier climate is an essential factor for an increase in sediment yield and fan aggradation. Over the last two decades vegetative cover change is increasingly cited as a prominent factor in triggering the onset of fan aggradation in deserts of southwestern North America, especially in analysis of fan deposition during the late Pleistocene–Holocene (LPH) climatic transition. To assess this assertion, we compiled paleobotanical and alluvial fan stratigraphy histories from 18 published studies broadly representing four different areas in the Mojave and northern Sonoran Deserts. Instead of focusing on chronology aggregation and comparison with global or regional climate change proxies forced by orbital parameters, we discriminated by altitudinal regions, looking for linkages between local vegetation change and alluvial deposition. Results indicate that the onset of extensive alluvial fan deposition (1) began well before changes in catchment vegetative cover, and (2) can occur during several possible combinations of vegetation change. The ambiguous relation between vegetation change and alluvial fan aggradation indicates that vegetation had a reduced role in LPH aggradation. Other factors, such as local storm intensity and water and sediment redistribution pathways on hillslopes, need to be considered in this analysis given the important role that the combined hillslope/alluvial system has on arid region ecosystem functions.

Alluvial fans are important paleoclimatic archives, that may record high-frequency climatic oscillations. However, climate signals may be overprinted or even be destroyed by autogenic processes caused by channel avulsion and lobe switching. Here we present new data from two different Late Pleistocene (MIS 3–2) alluvial fan systems in northern Germany and compare these systems to experimental alluvial fans and other field examples. The selected fan systems formed under similar climatic and tectonic conditions, but differ in size, type, and drainage area allowing to estimate the role of climate and autogenic controls on flow processes, facies architecture, and fan-stacking patterns. Luminescence dating is used to determine the timing of fan onset and aggradation. Fan onset occurred in response to climate change at the end of MIS 3 when temperatures decreased and periglacial climate conditions were established in northern central Europe. A related increase in sediment supply and strongly variable precipitation patterns probably promoted fan formation. The major period of fan aggradation was approximately between 33 and 18 ka, followed by fan inactivity, abandonment, and incision during the Lateglacial. The highest aggradation rates occurred during the early stage of fan building, when up to 35 m thick sediment accumulated within a few thousand years. Sand-rich, sheetflood-dominated fans are related to larger, low-gradient fan catchments. Steep depositional fan slopes (5°–17°) and short-lived high-energy floods promoted supercritical flow conditions. Well sorted, sediment-laden, rapidly waning flows favored the deposition and preservation of supercritical bedforms and allowed for the aggradation of stable antidunes. Steep, dip-slope catchments enhanced stream gradients and promoted the transport of coarser sediments. These fans have lower gradient slopes (2–6°) and are dominated by channelized flows, alternating with periods of unconfined sheetfloods. Meter-scale coarsening upward successions, characterized by sandy sheetflood deposits at the base, overlain by multilateral or smaller single-story gravelly channel fills may be related to high-frequency climatic fluctuations or seasonal fluctuations in water and sediment supply. These coarsening-upward successions are commonly bounded by a paleo-active layer, from which ice-wedge casts penetrate downwards. The comparison to experimental fans and other field examples implies that the recurrent pattern of multistory, multilateral and single-story channel bodies with a lateral offset to vertical stacking pattern most probably was controlled by autogenic switch in an avulsion-dominated system. The change in deposition from alluvial-dominated processes to aeolian sedimentation with minor alluvial influences during the Lateglacial records alternation of dry and ephemeral wetter phases that are related to rapid climatic variations. The main phase of aeolian sand-sheet deposition probably correlates with Heinrich event H1 between approximately 18–16 ka and reflects sedimentation in response to aridification and high mean wind speeds.

Reach‐scale widening of channelized rivers can locally reactivate morphodynamic processes and increase habitat heterogeneity. Following process‐based restoration principles, construction efforts are often limited to bank protection removal and local bank erosion initiation. Despite significant advances in our understanding of fluvial processes, the influence of sediment supply on widening morphodynamics is still inadequately understood. Using mobile‐bed laboratory experiments, we studied the morphological development of river widenings in channelized subalpine gravel‐bed rivers in response to different sediment supply levels. The initial setup included a channel with a longitudinal slope of around 1% and a floodplain on one side, which was 25 channel widths long and three wide. After removing the fixed bank, the floodplain was available for erosion and channel shifting. Seven experimental series with sequences of steady bed‐forming discharge and unsteady larger floods were conducted. Sediment was supplied at rates of 20%, 60%, 80%, and 100% of the initial channel's transport capacity. The laboratory experiments were combined with two‐dimensional hydrodynamic numerical simulations to closely track both the morphological development and the resulting flow conditions. Sediment supply was found to be a critical driver of morphodynamic activity in river widenings, with sediment supply rates close to channel transport capacity promoting active channel widening, sediment relocation, and lateral channel‐floodplain connectivity. In contrast, low sediment supply maintained a stable, single‐thread channel disconnected from its floodplain. Further, our results indicate that an increase in sediment supply or structural measures triggering local flow deflection can reactivate morphodynamic processes in previously sediment‐starved reaches after a transition period of several years to decades. Our results highlight the fundamental importance of sediment transport in reach‐scale river restoration.

A systematic study of floodplains, terraces, and alluvial fans in the Republican River valley of south-central Nebraska provided a well-dated, detailed reconstruction of late Quaternary landscape evolution and resolved outstanding issues related to previously proposed Holocene terrace sequences. Stable carbon isotope (δ13C) values determined on soil organic matter from buried soils in alluvial landforms were used to reconstruct the structure of vegetation communities and provided a means to investigate the relationships between bioclimatic change and fluvial activity for the period of record. Our study serves as a model for geomorphological and geoarcheological investigations in stream valleys throughout the central Great Plains and wherever loess-derived late Quaternary alluvial fans occur, in particular.Holocene alluvial landforms in the river valley include a broad floodplain complex (T-0a, T-0b, and T-0c), a single alluvial terrace (T-1), and alluvial fans that mostly grade to the T-1 (AF-1) and T-0c (AF-0c) surfaces. Remnants of a late Pleistocene terrace (T-2), mantled by Holocene (Bignell) loess, are also preserved, and some Holocene alluvial fans (AF-2) grade to T-2 surfaces.Radiocarbon ages suggest that the T-1 fill and AF-1 fans aggraded between ca. 9000–1000 yr B.P. Hence, nearly all of the Holocene alluvium in the river valley is stored in these landforms. Sedimentation, however, was interrupted by several periods of landscape stability and soil formation. Radiocarbon ages from the upper A horizons of buried soils in the T-1 and AF-1 fills, indicating approximate burial ages, cluster at ca. 6500, 4500, 3500, and 1000 yr B.P. Also, based on the radiocarbon ages, the T-0c fill and AF-0c fans were aggrading between ca. 2000–900 yr B.P. Given that the T-0c fill and upper parts of the T-1 fill were both aggrading after ca. 2000 yr B.P., we suggest that the T-1 surface was abandoned between ca. 4500–3500 yr B.P., but subsequent aggradation of both the T-1 and T-0c fills occurred due to large-magnitude flood events during the late Holocene.The δ13C data indicate a shift from ∼40% C4 biomass at ca. 6000 to ∼85% at ca. 4500 yr B.P. We propose a scenario where (1) a reduction in C3 vegetation after 6000 yr B.P. destabilized the uplands, resulting in an increase in sediment supply and aggradation of the T-1 fill and AF-1 fans, and (2) the establishment of C4 vegetation by ca. 4500 yr B.P. stabilized the uplands, resulting in a reduction in sediment supply and subsequent incision and abandonment of the T-1 and most AF-1 surfaces. The proposed timing and nature of landscape and bioclimatic change are consistent with regional records from the central Great Plains.

River terraces and alluvial fans: The case for an integrated Quaternary fluvial archive

Fluvial landforms reflect a balance between tectonics, climate, and their interaction through erosion and sediment deposition. The occurrence of thick valley fills straddling the major Himalayan rivers testify an imbalance between sediment supply and river transport capacity. Whether the aggradation is related to enhanced sediment supply or reduced stream capacity is a matter of debate. The changes in runoff can qualitatively be determined from the paleoclimatic records but the changes in hillslope sediment supply are more difficult to measure and often remain speculative. In-situ produced cosmogenic nuclide inventories in fluvial sediments provide an estimate of catchment-averaged erosion rates. When applied to chronologically-constrained valley fill sediments, this method has the potential to provide paleo-erosion rate and, by implication, sediment discharge from the catchment hillslopes. The paleoerosion rate data in conjunction with the chronology of valley aggradation and paleoclimatic proxy records would allow assessing the impact of monsoon rainfall change on both the hillslope erosion rates and transport capacity of streams. We have applied this approach to the ~90-m thick Beas River valley fills that are exposed near the town of Kullu in Himachal Pradesh, northwestern India. Here, we present preliminary sediment depositional ages determined using the OSL and IRSL methods. Our new luminescence ages suggest that the aggradation of exposed deposits occurred between ̴ 155± 36.99 ka and 58.61± 12.98 ka. These ages when compared with other Himalayan River valleys, indicate a much older and prolonged phase of aggradation. We speculate that the observed discrepancy in depositional ages indicates that either the deposition began significantly earlier in the Beas river valley pointing towards the diachronous valley filling within the Himalaya or the river has incised at a comparably faster rate, resulting in the excavation of older valley fill deposits. We also observed a potential linkage between the terrace formation and monsoon variability where the existing aggradation phase correlates well with the higher rainfall trend when compared with the existing paleoclimate records. The results from our study are in well agreement with already existing depositional age models from other river valleys of Himalayas. 

Large-scale dam removal on the Elwha River, Washington, USA: Source-to-sink sediment budget and synthesis

Recent climatic warming and associated glacial retreat may have a large impact on sediment release and transfer in Alpine river basins. Concurrently, the sediment transport capacity of many European Alpine streams is affected by hydropower exploitation, notably where flow is abstracted but the sediment supply downstream is maintained. Here, we investigate the combined effects of climate change and flow abstraction on morphodynamics and sediment transfer in the Borgne River, Switzerland. From photogrammetrically derived historical Digital Elevation Models (DEMs), we find considerable net aggradation of the braided river bed (up to 5 m) since the onset of flow abstraction in 1963. Reaches responded through bed level steepening which was strongest in the upper most reach. Widespread aggradation however did not commence until the onset of glacier retreat in the late 1980s and the dry and warm years of the early 1990s. Upstream flow intake data shows that this aggradation coincided with an increase in sediment supply, although aggradation accounts for no more than 25% of supplied material. The remainder was transferred through the studied reaches. Estimations of bed load transport capacity indicate that flow abstraction reduces transport capacity by 1–2 orders of magnitude. While residual transport rates vary with morphological evolution, they are in the same order of magnitude as the sediment supply rates, which is why significant transport remains. However, the reduction in transport capacity makes the system more sensitive to short‐term (annual) changes in climate‐driven hydrological variability and climate‐induced changes in intake management and sediment delivery rates.

This chapter contains sections titled: Introduction Stream Power Analysis of River Longitudinal Profiles Profile Analysis of the Upper Noyo River Basin, California A Coupled Bedload Transport and Bedrock Incision Model Discussion Conclusions Notation

When water and sediment supply to a river change, the short‐term channel response will be a combination of adjustments in bed grain size (texture) and the accumulation or evacuation of sediment in the channel (topography). This response is well documented for gravel bed rivers, but little attention has been given to sand bed rivers, the focus of this paper. If the channel response is predominantly textural, subsequent changes in channel geometry may be minor. If the channel response is predominately aggradation or degradation, long‐term changes in channel dimension, planform, and slope may occur. Here, we use a mixed‐size morphodynamic model to explore the interaction between the textural and topographic responses to an increase in sediment supply in sand bed rivers. First, we consider how the steady‐state transport condition varies as a function of sediment supply rate and grain size. We then evaluate the path to steady‐state using numerical morphodynamic experiments. We find that bed aggradation in response to an increase in sediment supply can be reduced, eliminated, or even reversed depending on the sediment supply grain size. The path to the new steady‐state condition involves two adjustment phases: the first is textural‐dominated, and the second is topography‐dominated. Under the common condition of an increased supply of finer sediment, rapid textural adjustments can produce a transport capacity that is a large fraction of the new supply rate long before the system has reached complete equilibrium. Under conditions of supply coarsening, the initial textural adjustment is less dominant, and aggradation is immediate.

Changing sediment sources in the Bay of Bengal: Evidence of summer monsoon intensification and ice-melt over Himalaya during the Late Quaternary

The development of alluvial fans is sensitive to environmental change and, thus, alluvial fans provide essential archives for reconstructing Quaternary paleoenvironmental conditions, particular climate, hydrology, and tectonics. Although alluvial fans have been studied across the globe for over a century, there is no unifying scheme/framework or model to consider their complete variety and mode of formation. By reviewing the global spatial and temporal range of alluvial fan types and data from selected key dryland regions, we are able to develop a conceptual scheme/framework for their geomorphology and formation, and thus aid in their application for Quaternary climate and environmental change studies. This approach suggests that there are three main regimes for alluvial fan geomorphology and formation: Type I) microscale mountain alluvial fans, small in size and extent (radius < a few 100 m); Type II) mesoscale (radius

The primary factors that control alluvial fan evolution still remain in question particularly for the Holocene. Holocene centennial- and millennial-scale climate fluctuations are relatively subtle and more frequent than those of glacial/interglacial transitions, therefore intrinsic factors such as rock type or basin size are hypothesized to moderate significantly the influence of Holocene climate and climate change on alluvial fan processes. Here, we examine variability in styles and rates of alluvial fan aggradation along a single mountain front that is characterized by basins of varying size and rock type (carbonate versus granite). Basin rock type is more closely correlated to variability in the episodic nature and magnitude of alluvial fan aggradation than is basin area. Bedrock physical and chemical weathering properties control sediment delivery to the piedmont and thus influence alluvial fan aggradation. We suggest that the particle size of grus produced by weathering of granitic rocks fosters sediment mobilization and alluvial fan aggradation during episodes of increased precipitation in the Holocene. Sediment mobilization during wetter climates is also possibly enhanced by drought-related fires and vegetation loss that occurred during preceding drier periods. In contrast, carbonate outcrops weather to both dissolved materials and clastic sediment and relatively rapid cementation of talus precludes its transportation out onto the piedmont under almost all Holocene climatic conditions. If the scale of past Holocene climate change is the closest analogy to current global change, this study documents some mechanisms by which different rock types can exert dramatically different effects on landscape response to those changes.

ABSTRACTThe southern foreland basin of the Alborz Mountains of northern Iran is characterized by an approximately 7.3‐km‐thick sequence of Miocene sedimentary rocks, constituting three basin‐wde coarsening‐upward units spanning a period of 106 years. We assess available magnetostratigraphy, paleoclimatic reconstructions, stratal architecture, records of depositional environments, and sediment‐provenance data to characterize the relationships between tectonically‐generated accommodation space (A) and sediment supply (S). Our analysis allows an inversion of the stratigraphy for particular forcing mechanisms, documenting causal relationships, and providing a basis to decipher the relative contributions of tectonics and climate (inferred changes in precipitation) in controlling sediment supply to the foreland basin. Specifically, A/S > 1, typical of each basal unit (17.5–16.0, 13.8–13.1 and 10.3–9.6 Ma), is associated with sharp facies retrogradation and reflects substantial tectonic subsidence. Within these time intervals, arid climatic conditions, changes in sediment provenance, and accelerated exhumation in the orogen suggest that sediment supply was most likely driven by high uplift rates. Conversely, A/S < 1 (13.8 and 13.8–11 Ma, units 1, and 2) reflects facies progradation during a sharp decline in tectonic subsidence caused by localized intra‐basinal uplift. During these time intervals, climate continued to be arid and exhumation active, suggesting that sediment supply was again controlled by tectonics. A/S < 1, at 11–10.3 Ma and 9‐6–7.6 Ma (and possibly 6.2; top of units 2 and 3), is also associated with two episodes of extensive progradation, but during wetter phases. The first episode appears to have been linked to a pulse in sediment supply driven by an increase in precipitation. The second episode reflects a balance between a climatically‐induced increase in sediment supply and a reduction of subsidence through the incorporation of the proximal foreland into the orogenic wedge. This in turn caused an expansion of the catchment and a consequent further increase in sediment supply. Copyright © 2013 John Wiley & Sons, Ltd.