Canadian Forest Service Publications

Multi-millennial streamflow dynamics in two forested watersheds on Vancouver Island, Canada. 2015. Brown, K.J.; Schoups, G. Quaternary Research 83(3):415-426.

Year: 2015

Issued by: Northern Forestry Centre

Catalog ID: 36057

Language: English

Availability: PDF (request by e-mail)

Available from the Journal's Web site.
DOI: 10.1016/j.yqres.2015.03.003

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Abstract

Holocene streamflow was reconstructed for two rivers on Vancouver Island, British Columbia, Canada in 500-yr intervals. The San Juan River watershed is located on the wetter western side of the island, whereas the Koksilah Riverwatershed is positioned on the drier eastern side. Both watersheds are forested. To reconstruct streamflow, temporal changes in precipitation (estimated using a pollen-based transfer function) and evapotranspiration were established for each watershed and integrated into a water balance model, calibrated using modern data. While seasonal streamflow variability was maintained throughout the Holocene, with greater flow in the winter relative to the summer, the amount of discharge has changed markedly through time. Lowest simulated flow occurred in the earliest Holocene, with low-flow conditions beginning earlier in the year and extending later into the fall. Such conditions may have inhibited salmon from using many of the smaller rivers in the region. Streamflow steadily increased throughout the early Holocene so that by ca. 6500 cal yr before present near-modern flow regimes were established. As climate changes in the future, the San Juan and Koksilah watersheds are expected to remain as pluvial hydroclimatic regimes, though with an extended season of low flow similar to conditions during the early Holocene.

Plain Language Summary

To understand how rivers and streams may change in the future, affecting their vital functions in the environment and the economy, it is important to understand how they have changed through time. The current research focuses on two watersheds on south Vancouver Island: San Juan, on the wetter western side of the island, and Koksilah, on the drier eastern side. To examine long-term trends in stream flow, we developed a computer model of the water balance for a drainage basin during our current interglacial period, which began 11,700 years ago. The model takes into account historical data on precipitation, radiation from the sun and temperature. The model shows that water flow in the basins has varied notably through time, and was lowest during a warm, dry interval around 10,000 years ago. During this period, low flow conditions began earlier in the year and ended later. Over time, the climate became wetter. About 6500 years ago, the rivers began to flow at the level they do today. This period may represent the time when salmon began to spawn in many of the smaller rivers in coastal British Columbia. This and other studies suggest that, in the future, the basins will continue to be dominated by rainfall, although low flow conditions may begin earlier and extend later in the year.