THE BIRTH OF THE HUMID ALLUVIAL FAN (DELTA) MODEL, PIONEERING RESEARCH OF JON C. BOOTHROYD IN SOUTHEAST ALASKA

In the 1960’s, source-to-sink alluvial sedimentology was in need of new and fresh ideas. On the “source end”, flume runs had revealed the hydraulic link between bed forms and flow characteristics and the SEPM S.P.# 12 elegantly outlined the connection between bed forms and primary sedimentary structures. Field studies of glacial outwash progressed slowly from descriptive geomorphology to quantitative process sedimentology that linked hydraulics, channel patterns and the sedimentary record.

On the “sink end” of the system were alluvial fans and deltas. Subaerial alluvial fans were studied mainly in tectonically-active arid to semi-arid regions with discussions of Davis’cycle of uplift and erosion vs. dynamic equilibrium, as well as the relative importance of sediment transport by debris flows vs. fluvial transport. Delta research was fueled by the petroleum industry; studies of outcrops, geophysical data, and cores. Four science communities (fluid mechanics, glaciofluvial geomorphology, alluvial fan geomorphology and marine deltas) were working independently, analogous to the blind men and the elephant. A scientific revolution was in order. T.S. Kuhn’s (1970) paper “The Structure of Scientific Revolutions” described science as characterized by long periods of problem-solving with steady knowledge accumulation, interrupted by brief intellectual crises. Sedimentology was ready for a revolution.

Jon Boothroyd’s studies of the Scott and Yana glacial outwash fans on the southeast Alaska documented the facies and the associated processes from the sediment source (glacial ice) to the sink (delta) building into the Gulf of Alaska. He integrated the relevant knowledge of small-scale aspects, such as hydraulics, fluvial geomorphology, sedimentological processes and the large-scale features deltas and alluvial fans into his humid alluvial fan model. Using local gradient, clast size, grain size and sorting, braid bar morphology, he modeled humid alluvial fans with unique characteristics of channel pattern and downstream trends. They have been recognized globally in modern and ancient context and perhaps rediscovered in this decade as DFS (Distributative Fluvial Systems). Boothroyd’s pioneering research sets a gold standard for thinking outside the box.