The influences of lateral ground-tilting on channel morphology and alluvial architecture

This thesis characterises and quantifies the influence of lateral ground-tilting on rivers in terms of: i) channel planform changes and migration style, ii) temporal and spatial patterns of channel movement, and, iii) the preserved alluvial architecture formed by aggrading fluvial systems. A dual field and physical modelling approach is used to examine the impact of tilting on river channels at a range of temporal and spatial scales. Fieldwork was undertaken on the Carson River, Nevada, USA which has been proposed as an example of avulsive downdip channel movement towards an active basin-bounding fault (Leeder, 1993). Froude scale models of both meandering and braided rivers were established in a stream table which could simulate aggradation and be tilted laterally. Quantification of the Holocene history of the Carson River, combined with independent dating of movements on the basin-bounding fault, allows the alluvial-tectonic relationships to be analysed. In addition to local tectonics, factors which may also affect channel movement such as climate change, regional periods of channel incision and intrinsic channel avulsion are investigated. The Carson River is shown to possess a more complex spatial relationship with fault movement and lateral tilting than that envisaged by Leeder (1993). Phases of channel onlap towards the locus of subsidence after fault events are separated by channel offlap during periods of tectonic quiescence. In addition, lag times between faulting and channel movement appear to be short. The preserved alluvial architecture modelled in flume experiments on aggrading braided streams is subdivided into a number of key depositional niches and their geometries quantified. A number of cross-stream and vertical variations in niche geometries can be attributed to the timing, magnitude and frequency of imposed lateral tilting, and confirm Leeder and Alexander's (1987) conceptual model of progressive downdip channel movement in response to lateral tilting. No unambiguous relationship between channel bend parameters and imposed lateral ground-tilting could be defined in the meandering river experiments because the rate and type of bend development masks the impact of tilting. However, the rate of channel movement towards the downtilted margin appears to increase after tilting and be directly related to tilt magnitude. A new model of progressive downdip channel migration in response to lateral tilting is presented which resolves the incompatibility of previous models. In addition, a criterion based on the rate of tilting is proposed in order to distinguish between channels that respond to tectonic tilting either by avulsion or by gradual downdip migration