The nature and timing of postglacial incision of Nicola River, British Columbia.

Nicola River, between Merritt and Spences Bridge in interior British Columbia (BC) has left behind a suite of river terraces as it incised its glacial valley fill. Terraces formed through episodic incision and aggradation throughout the paraglacial period. Through optical dating terrace formative ages were elucidated and valley incision rate estimated. Calculation of an estimated incision rate permitted rough correlation to periods of known climate change and the exploration of climate as a formative driver of terrace formation. Accepted ages in this study range between 8.53 ± 1.06 to 2.76 ± 0.51 ka, consistent with other terrace formative ages in the region. Since 8.53 ± 1.06 ka, Nicola River has incised through 54.5 m of glacial valley fill to its present level. Present day Nicola River is still incising through glacial fill though bedrock is exposed in some canyonised reaches. Some optical dating samples returned overestimated ages. These are thought to be associated with incomplete bleaching of mineral grains. Incomplete bleaching is likely linked to valley size, presenting limited bleaching opportunity. The proximity of Nicola River to valley walls may also incorporate unbleached grains into sediments. Average valley incision rate over the last ∼8.5 ka is 6.5 mm/a but varies between 1.5 and 9.5 mm/a. The fastest rates occur during the Holocene Mesothermic Interval (HMI), between 7.5-4 ka. This period is associated with cooler and wetter conditions but also the development of forests. These factors increased slope stability and stream power whilst upland sediment supply decreased. This investigation is the first detailed study of Nicola Valley terraces and their associated formative ages.