Adrian M. Harvey

Effective timescales of coupling within fluvial systems

This paper presents a review of the coupling concept in fluvial geomorphology, based mainly on previously published work. Coupling mechanisms link the components of the fluvial system, controlling sediment transport down the system and the propagation of the effects of base-level change up the system. They can be viewed at several scales: at the local scale involving within-hillslope coupling, hillslope-to-channel coupling, and within-channels, tributary junction and reach-to-reach coupling. At larger scales, coupling can be considered as zonal coupling, between major zones of the system or as regional coupling, relating to complete drainage basins. These trends are illustrated particularly by the examples of hillslope-to-channel coupling in the Howgill Fells, northwest England, badland systems in southeast Spain, alluvial fans in Spain, USA and UAE, and base-level-induced dissection of Neogene sedimentary basins in southeast Spain. As the spatial scales increase, so do the timescales involved. Effective temporal scales relate to magnitude and frequency characteristics, recovery time and propagation time, the relative importance changing with the spatial scale. For downsystem coupling at the local scale, the first two are important, with propagation time increasing in importance in larger systems, especially in those involving upsystem coupling related to base-level change. The effective timescales range from the individual event, with a return period of decades, through decadal to century timescales for downsystem coupling, to tens to hundreds of thousands of years for the basinwide response to base-level change. The effective timescales influence the relative importance of factors controlling landform development.

Coupling between hillslopes and channels in upland fluvial systems: implications for landscape sensitivity, illustrated from the Howgill Fells, northwest England

Abstract The sensitivity of upland fluvial systems depends on the magnitude and frequency of sediment and flood producing events, modified by the internal coupling characteristics of the system. This paper assesses the role of hillslope/channel coupling for the sensitivity of upland geomorphic systems, using evidence from a 30-year monitoring programme of geomorphic change in the Carlingill valley, Howgill Fells, northwest England. In the hillslope zone, there is little sediment supply to the stream system. Locally, slope failures occur in response to extreme events (six such events in 30 years within the ca. 6 km 2 Carlingill valley). In the footslope coupling zone, basally induced gullies are major sediment sources to the stream. Sediment-production events occur ca. 30 times per year, feeding sediment to basal debris cones. Stream floods which can entrain these sediments occur once every ca. 2–5 years. Stream channel morphology is adjusted to this regime. Downstream of gullies, channels are wide and braided; elsewhere they are narrow and single-thread. As the gullies develop, however, this coupling weakens and the eroding slopes eventually stabilise by revegetation. Over the 30-year monitoring period, there has been a progressive trend towards gully stabilisation, and an associated reduction in channel instability. Massive destabilisation may occur in response to rare extreme flood events. Such an event (return period >100 years) occurred in neighbouring Langdale and Bowderdale valleys in June 1982. That event destabilised the system, causing slope failures, fan deposition, and in some places, a switch in channel style to wide braided channels. Since 1982 there has been a progressive recovery by slope stabilisation and single-thread sinuous channels have become reestablished. A different style of extreme event occurred in Carlingill in October 1998, in response to the wettest week in the 30-year period. A slope failure fed debris flows 400 m downslope, almost coupling with the channel system. Future climatic change could render the system prone to destabilisation, through either extreme flood events or through major slope failures.

Sedimentologic and geomorphic variations in storm-generated alluvial fans, Howgill Fells, northwest England

In June 1982, a storm with a return period greater than 100 yr, but lasting less than 2.5 hr, destabilized hillslopes and produced a suite of geomorphologically and sedimentologically diverse alluvial fans. Thirteen major fans were deposited at the tributary junctions between small ( 2 ) catchments and two north-flowing; headwater streams of the River Lune, northwest England. Storm-generated fans spread over or became inset into older stable fans and produced localized vertical accretion of as much as 3 m and lateral accretion of as much as 100 m. Sedimentary processes operating during deposition involved debris flow, transitional flow, and streamflow. Six facies types are recognized on the basis of depositional topography, sedimentary texture and fabric, and matrix content: viscous debris flow (D1), dilute debris flow (D2), transitional flow (T1), fluvial bars and lobes (S1, S2), and fluvial sheet gravels (S3). Regionally, streamflow deposition prevails over debris-flow deposition, and type S3 facies has the greatest areal extent. Temporal and spatial variations in facies deposition during the storm, however, resulted from water:sediment ratio variations. Fan deposition involved an early phase of debris flow to transitional flow due to large inputs of sediment from hillslope failures. This was followed by a systematic change to more dilute conditions, resulting in streamflow deposition and eventually in channel incision. A significant amount of geomorphic work and complex variations in sedimentary processes during the storm resulted, in part, from extensive overland flow and hillslope destabilization. Discriminant analyses indicate that catchment size, channel gradient, and percentage of area eroded during the storm controlled whether debris-flow or streamflow facies dominated a fan sequence. Smaller, steeper catchments had a greater percentage of the area yielding sediment and are dominanted by debris flows, whereas larger catchments produced more runoff resulting in dilution and streamflow. The results indicate that the facies sequences and fan entrenchment in the Howgill Fells, which are typically considered products of longer term climatic change or tectonics in other localities, are here primarily affected by thresholds related to catchment geomorphology, by the type of sediment available, and by the position within the storm cell.

The role of base-level change in the dissection of alluvial fans: case studies from southeast Spain and Nevada

Abstract Tectonics, climate and base level are the underlying controls of alluvial fan development. On many fans base level is stable, but where base-level change occurs it may itself be a response to tectonics or climatic change. Tectonically induced base-level change will be temporally independent of climatic change, and will show spatial variability depending on the rate of propagation of tectonically induced regional dissection. This is illustrated by the Tabernas fans, southeast Spain. The timing of climatically induced base-level change follows the timing of climatic change itself, and will show more consistency of spatial relationships within groups of fans affected. Alluvial fans which toe out in coastal zones or at lake shorelines may be influenced by climatically controlled base-level changes. Conventionally, a fall in base level would be expected to cause dissection of the distal fan zone. However, the reverse may occur, with fan progradation taking place in response to a base-level fall, and dissection occurring in response to a base-level rise. Two contrasted case studies are considered, based in part on previously published work, from dry-region alluvial fan zones, the coastal Cabo de Gata fans in southeast Spain and the Stillwater and Cold Springs fans in Nevada, USA. In these areas, base-level changes occurred in response to late Quaternary eustatic changes of sea level, and changes in the levels of pluvial lakes, respectively. Within both areas there are fans not affected by base-level change (Cabo de Gata, west-coast and inland fans; Cold Springs fans), and others where base-level change has been important in fan evolution (Cabo de Gata, east-coast fans; Stillwater fans). The differences in geomorphic regime between those fans influenced by base-level changes and those not are expressed in morphometric contrasts and different fan profile and plan characteristics. Within those fans influenced by base-level changes, the different mechanisms have influenced the location and elevation of the zone of incision. The differences can be attributed in part to the temporal interactions of base-level change with climatically induced changes in sediment supply, and in part to differences in gradients of the foreshore exposed by the falling sea or lake base levels.

The impact of Quaternary sea-level and climatic change on coastal alluvial fans in the Cabo de Gata ranges, southeast Spain

Conventionally, a fall in base level is seen as stimulating incision into the distal zones of alluvial fans. In the Cabo de Gata ranges of southeast Spain evidence exists to the contrary. Two sets of Quaternary coastal alluvial fans demonstrate the interaction between climatically-driven variations in the supply of sediment and eustatically-driven changes in base level. The fans are supplied from Miocene volcanic terrain within which no evidence can be found for major tectonic deformation during the period of fan development. The evolution of the east-coast fans has been affected by variations in sediment supply and changes in sea level. The west-coast fans were buffered from the effects of changes in sea level by coastal barriers. Three phases of past sedimentation can be identified on the fans. These can be differentiated on the basis of field observations of soil profiles (particularly colour of the B horizons and accumulation of CaCO3), and laboratory analyses of sequential iron oxide extractions and magnetic mineral properties. The two earlier (major) sedimentation phases were coincident with global glacials (>ca. 135 ka and ca. 85–10 ka, based on the stratigraphy and uranium/thorium dating of the coastal sediments). High sea levels during the intervening interglacial and during the Holocene caused erosion of the distal zones of the east-coast fans which led to channel incision into the fan surfaces. On the west-coast fans no such incision occurred, simply proximal incision by small fanhead trenches. The youngest (relatively minor) phase of fan sedimentation has occurred during the Holocene. These contrasting contexts have produced differing styles of fans, with telescopic fan morphology on the east-coast and stacked morphology on the west-coast fans. The differences are reflected in the fan profiles, with steeper gradients dominating the east-coast fans, and extensive lower gradient distal surfaces on the west-coast fans. Fan morphometry, based on analysis of the residuals from drainage area to fan area and gradient regressions, also differentiates between the fan contexts. The fan building phases appear to be controlled proximally by climatically-driven pulses of sediment supplied to the fans. These occurred during global glacials coincident with low sea levels, and caused fan progradation onto the exposed foreshore. The intervening global interglacials were times of little fan sedimentation, and on the east coast, where high sea levels were able to erode the fan toes, deep through-fan dissection ensued.

Response of alluvial fan systems to the late Pleistocene to Holocene climatic transition: contrasts between the margins of pluvial Lakes Lahontan and Mojave, Nevada and California, USA

Abstract Dated shorelines of late Pleistocene pluvial Lakes Lahontan (Great Basin Desert, northwest Nevada) and Mojave (Mojave Desert, eastern California) provide timelines for the assessment of alluvial fan sedimentation at the lake margins during the late Pleistocene to early Holocene. Two sets of alluvial fan systems have been mapped: the Stillwater fans, feeding Lake Lahontan; and the Zzyzx fans, feeding Lake Mojave. Their contrasting morphologies suggest different responses of the two fan systems to late Pleistocene to early Holocene climatic change. At the time the Stillwater fan systems underwent minimal sedimentation, with the catchment hillslopes apparently stable. The Zzyzx fans experienced major changes in water and sediment supply from the catchment hillslopes. There was a major phase of hillslope debris-flow activity, followed by fanhead trenching and distal fan progradation. Both areas were wetter and colder in the late Pleistocene than they are today, but during the transition to the Holocene the Zzyzx area was more likely to experience intense rains associated with the monsoonal penetration of warm moist tropical air into the Southwest. Vegetation reconstructions for the late Pleistocene to the early Holocene suggest that catchment hillslopes in the Mojave supported a desert shrub vegetation, but those in the Stillwaters supported juniper woodland and grasses at low elevations and pine at higher elevations. Contrasts in hillslope vegetation cover together with storm activity may account for the different responses of the alluvial fans to climatic change during the Pleistocene to Holocene climatic transition. After the falls in lake levels of Lakes Lahontan and Mojave in the early Holocene, both areas underwent aridification, resulting in reductions in hillslope vegetation cover. Increased storm runoff led to fanhead trenching and distal progradation of the alluvial fans. Variations in fan style at that time may relate primarily to base-level conditions resulting from different gradients on the exposed lake shores.

Alluvial fans: geomorphology, sedimentology, dynamics — introduction. A review of alluvial-fan research

This volume presents a series of papers on the geomorphology, sedimentology and dynamics of alluvial fans, selected from those presented at the ‘Alluvial Fans’ Conference held in Sorbas, SE Spain in June 2003. The conference was sponsored primarily by the British Geomorphological Research Group and the British Sedimentological Research Group, both organizations affiliated to the Geological Society of London. It is some time since an international conference has been held that was exclusively devoted to the geomorphology and sedimentology of alluvial fans. The previous such conference was that organized by Terry Blair and John McPherson in 1995, and held in Death Valley, a classic setting for alluvial fans (Denny 1965; Blair & McPherson 1994a). Although many of the papers presented there have since been published, no dedicated volume on alluvial fans as a whole resulted from that meeting, so even longer has elapsed since there has been a specific publication devoted wholly to a series of papers on the geomorphology and sedimentology of alluvial fans (Rachocki & Church 1990). South-east Spain was chosen as the venue for this conference, partly for logistic reasons and partly because it is a tectonically active dry region within which there is a wide range of Quaternary alluvial fans. These fans exhibit differing relationships between tectonic, climatic and base-level controls (Harvey 1990, 2002a, 2003; Mather & Stokes 2003; Mather et al. 2003), core themes in consideration of the dynamics of alluvial fans. An emphasis within the previous alluvial fan literature has been on fans within

Desert pavement characteristics on wadi terrace and alluvial fan surfaces: Wadi Al-Bih, U.A.E. and Oman

Abstract In arid mountain areas, the dating and correlation of alluvial depositional surfaces is often uncertain. Especially in regions where the geomorphology is not well known, surface modification by the development of soil and desert pavement may allow the correlation of geomorphic surfaces and estimation of at least their relative ages. Pleistocene wadi terraces and associated alluvial fans occur in Wadi Al-Bih, U.A.E. and Oman, for which correlations and age relationships are not known. Three age-related groups of fans and terraces have been identified and mapped on the basis of their morphostratigraphic relationships. Deposition of the oldest terrace sediments and associated fans followed a long period of sustained incision after Miocene uplift of the region. The younger two groups of terraces and fans are inset within the older group. To identify the gross effects of pavement development, comparisons have been made between terrace surface and subsurface particle-size distributions. The older terraces have finer surface sediments and a greater contrast between finer surface and subsurface sediments than the younger terraces. This reflects the degree of pavement development. Particle size on the fan surfaces is comparable with that on the equivalent terrace surfaces. Criteria for the classification of pavements were developed based on clast fracturing and angularity, size, sorting, packing, and surface texture, from which a simple index of pavement development has been derived. Other properties, rock varnish and weathering characteristics, were also recorded; but these proved to be less discriminatory than pavement characteristics. The pavement data have been augmented by observations on soils. Detailed studies of pavements on terraces (8 sites, 12 samples covering the three age groups) and fans (5 sites, 10 samples covering the three age groups) allow differentiation between age-groups. The three terraces show three different age-related pavement types, expressed by differences in the pavement development index. Weakly-developed pavements (little fracturing, sub-rounded clasts, some modification of the depositional fabric, incipient soil development, stage I CaCO3 accumulation) occur on the youngest terrace and fan surfaces. Moderately-developed pavements (clast fracturing, sub-angular clasts, moderate sorting and packing, deeper soil development, stage II CaCO3 accumulation) occur on the middle terrace and fan surfaces. Well-developed pavements (complete clast fracturing into small angular fragments, mature sorting and packing of the pavement surface, deep soil development with strong horizonation, stage III CaCO3 accumulation) occur on the highest terrace and oldest fan surfaces. There are minor differences between the youngest pavements on terraces and fans, which reflect initial sedimentological differences. These differences become less as the pavements develop. On the basis of comparative studies, the oldest terrace is estimated to date from sometime prior to ca. 100 ka BP, the second terrace and the most extensive fan surface from the Late Pleistocene, and the youngest terrace and fan phase from the Latest Pleistocene or Early Holocene.

Process interactions, temporal scales and the development of hillslope gully systems: Howgill Fells, northwest England

Abstract Hillslope gully development is often controlled by interactions between multiple processes, rather than by individual processes. Such interactions may have both spatial and temporal characteristics. A twenty-year monitoring program of hillslope gullying in the Howgill Fells, northwest England, has identified the roles of both on-slope interactions and slope/stream coupling interactions, which can be related to several temporal scales. On-slope interactions are primarily seasonal, whereas those related to coupling produce a cyclicity over timescales of up to several years. Over the longer term (decades or more) progressive changes in gully morphology modify the nature of both sets of interactions, and through negative feedback mechanisms may ultimately lead to gully stabilisation. Gully development is controlled by the rates of headwall recession and the basal incision/stabilisation behaviour. After basal stabilisation, vegetation encroachment onto the eroding slopes takes place faster than headwall retreat, resulting in a finite age and limiting size for gully development. A simple model based on the current rates of development adequately describes the morphology of modern gullies but is inappropriate when applied to an older generation of stabilised gullies. This suggests that there have been changes in sediment generation and removal rates during the late Holocene.

Quantifying long-term catchment changes of alluvial fan systems

In mountain areas affected by uplift, significant reorganization of drainage networks can occur through river capture. This modification can dramatically affect sediment flux and routing into adjacent sedimentary basins. It is thus important to obtain information on rates and direction of changes in catchment areas in such environments. This can be achieved where the original, precapture catchments can be compared with the postcapture scenario. This paper examines three modern mountain catchments from southeast Spain that were affected by river capture, and that fed relict Pliocene–Pleistocene alluvial fan systems. Morphometric data have been collated for complete Quaternary mountain catchment piedmont fan systems. These data are used to establish regressions to determine catchment area and basin relief from a combination of fan characteristics (area and gradient). These relationships are then used to predict the original catchment characteristics for the three Pliocene–Pleistocene relict fans and compared with the modern catchments to quantify the magnitude and direction of changes, which have affected the drainage basins since fan abandonment in the early Pleistocene. The figures suggest that the catchment areas are responding to regional uplift, most significantly by drainage net expansion facilitated by river capture rather than lowering of surface relief. As a result, sediment routing into the sedimentary basins has been radically altered.