GEOMORPHIC MAPPING AND ANALYSIS OF GLACIAL LANDFORMS TO UNDERSTAND THE DEVELOPMENT AND DEFINING CHARACTERISTICS OF ‘GLACIAL’ LAKE SPEIGHT IN THE MIDDLE WAIMAKARIRI RIVER OF THE SOUTHERN ALPS, NEW ZEALAND

The Southern Alps of New Zealand have been affected by multiple extensive glaciations, the most recent experienced its local Last Glacial Maximum (LGM) 26,500 years ago. In particular, the Middle Waimakariri River now runs through a valley with a variety of landforms with fluvial, denudational, limnic and glacial origins. Those landform features have been interpreted in the past decades, but little available numerical age dating and geospatial data make it difficult to constrain their development over time. This is especially in relation to the ‘glacial’ Lake Speight paleo-lake, whose damming and drainage mechanisms have been hypothesized to be glacial more than half a century ago. No modern analogues, alternative models, or chronological constraints have, however, been examined yet. Therefore, landform features from this event that occurred uniformly across the landscape, such as paleo-lake wave cut benches on the valley sides, are key to understand regional landform chronologies and reassess previous assumptions. Using landforms to understand the creation and drainage of this lake is important to understanding both glacial timing and potential geohazards.

Using morphometric observations and images from the field, grain size curves of sediment samples, and orthorectified imagery utilized in ArcGIS, this study provides a detailed geomorphological map of the Middle Waimakariri. This map utilizes field mapping techniques as a tool to understand the timing of landform development, reassessing proposed mechanisms of damming and draining established for ‘glacial’ Lake Speight in the literature, and patterns of paleo-lake ridges. This study concludes that ‘glacial’ Lake Speight was not glacial in origin, but rather a mass-movement dam created shortly after the glacier receded from the terminal moraine region, following a period of alluvial accumulation. The slow pace of drainage deduced from the sequence of multiple paleo-shorelines is explained by the stability of underlying bedrockpre-existing sediment. This drainage pattern and mass movement dam stability have important implications for mass-movement dams as global warming and increasing glacial melting create an increased need for proglacial lake geohazards assessments.