GEOMORPHOLOGICAL EVIDENCE AND NUMERICAL MODELLING OF A LATE-GLACIAL MEGAFLOOD, SOUTH-CENTRAL BRITISH COLUMBIA

A large glacial lake formed behind a series of ice dams in the Nicola Valley, south-central British Columbia, during deglaciation of the last Cordilleran Ice Sheet. The most voluminous stage of glacial Lake Nicola was the Hamilton 2 stage, at 160 km³; ice recession allowed 125 km³ to drain to the succeeding stage was via Campbell Creek, a 30 km stream flowing northwards into the South Thompson River. The southern half of the Campbell Creek valley is a series of long, narrow lakes situated in a steep-sided valley incised into glacial drift. In the northern portion of the valley, an underfit stream flows 10 km through a meandering canyon. This canyon contains several large cobble-boulder deposits, interpreted as point bars, pendant bars, flow separation bars and run-up bars. The canyon opens to a large deltaic deposit, prograding to the surface of a paleolake in the South Thompson Valley. The delta’s topsets also comprise very coarse gravel, containing boulders exceeding 2 meters in diameter and large diamicton intraclasts. Delta foresets exhibit a complex assemblage of upper flow regime sand and gravel, including plane beds, antidunes and sigmoidal cross-beds, aggrading over bedded silt; foresets are sharply overlain by lake silt.

These lines of evidence are consistent with megaflood drainage of the Hamilton 2 stage of glacial Lake Nicola through Campbell Creek. Coarse gravel bars record dynamics of a high-magnitude flow within the canyonized reach. The delta records rapid sedimentation under fast, shallow flow as the flood entered the lower lake basin. The incised southern portion of Campbell Creek formed by head cutting towards the lake basin during drainage.

A range of empirical and semi-empirical paleohydraulic formulae using transported boulders and spillway cross-sections, and high resolution (25 m) three-dimensional flood modelling, estimate shear stress of 1-2 x 10³ Pa, stream power of 5-15 x 10³ W m^-2, velocities of around 10-45 ms^-1, and peak discharge of between 10⁵ and 5 x 10⁶ m³s^-1. These estimates and the geomorphic record show excellent consilience. This study highlights the potential that megafloods may be evident in the geologic record at the local scale, and informs risk and hazard assessment of ice dammed lake drainage in deglaciating regions today.