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MICHAEL C. ROBERTS
Department of Geography, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, Canada V5A 1S6 (e-mail: [email protected])
Some 250 years ago, near the town of New Aiyansh in northwestern British Columbia, lava flowed down the narrow valley of the Tseaux River and then spread out over the floodplain of the Nass River causing catastrophic changes to the geomorphology of the area (Figure 1). Two villages of the Nisga'a people, located on Nass River floodplain, were destroyed by the flow. The event took place over a period that lasted probably no more a few days or, at the most, a couple of weeks (Wuorinen 1976).
This paper describes how the Aiyansh lava flow radically changed the fluvial geomorphology of the Tseax and part of the Nass River valleys. The lava blocked and partially infilled the Tseax valley and thereby created lakes, diverted channels, formed multi-channeled streams, covered the lower reaches of alluvial fans, and forced the Nass to abandon its floodplain and to flow in a confined bedrock channel. In places, there is no longer surface flow because the Tseax River flows entirely within the lava flow.
The basaltic lava flow occupies portions of the Tseax and Nass Valleys in the Nass Depression physiographic region, located between the Coast and Skeena Mountains of northwestern B.C. It is located at the southern end of the Stikine Volcanic Belt. The Tseax River flows northward into the Nass River, joining it just west of New Aiyansh (Figure 1). The Nass River valley widens at this location forming an extensive floodplain that is now buried beneath the lava flow. The Tseax drainage basin was glaciated and has a characteristic alpine geomorphic suite of hanging valleys, cirques, horns, aretes and cirque glaciers in its higher reaches. High-energy streams draining the mountain slopes frequently form alluvial fans (debris cones) in the valley bottoms. Debris flows and avalanche tracks are common on the mountain slopes.
The lava flow
Composition: The Aiyansh lava flow consists of black alkali basalt distinguished by the presence of microskeletal olivine phenocrysts in a groundmass of plagioclase. The petrology of the flow is uniform from the cone to the flow's most distal edge, although its texture and crystallinity are highly variable (Sutherland Brown 1969).
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