Lynn Canyon Stream Channel Changes and Glaciation Evidence

Introduction to Lynn Canyon's Stream Environment

The study of the canyon is best conducted — as in this case — during winter, spring, and summer, when water levels are low and most features are exposed for observation. Safety within the canyon is also more assured during these seasons than during heavy rains. In autumn, heavy rainfall raises water levels significantly, and the melting of peak snowfall in spring can similarly elevate water levels, ultimately obscuring some of the lower-elevation observation points.

Changes in the Stream Channel

This study focuses on the river that flows along the bed of Lynn Canyon, examining its physical features, the direction and changes in its flow, and the evidence that reveals the history of changes the river has undergone in terms of direction, force, and scale over time.

The stream within the canyon has shifted its channel repeatedly over the years due to the presence of large boulders. In some places, the stream is observed to flow around elongated boulder ridges within the channel, ridges formed during higher-flow stages. Across and upstream, there is a boulder surface that sustains a forest of 10-meter trees along the east bank of the stream. Further downstream, the river flows around an island with larger trees, and the banks consist of a lower layer of boulders covered by a layer of sand.

The forest along the stream is a mosaic of patches made up of trees of different ages. The age of each tree patch indicates the date on which that section of the streambed was last an active bar in the channel of Lynn Creek. The river channel is also observed to shift its position over time; at intervals, the river abandons its channel and migrates to a new position. This is evidenced by the variable ages of trees along the stream, and in particular along the floodplain (Bob T. & John C., n.d.).

Erosion, Flooding, and Sediment Transport

There is also evidence of bank erosion, visible in the steep, unvegetated banks near which the stream flows and in the trees that have fallen into the river as a result of bank undercutting. The absence of vegetation in certain areas further indicates that the river once flowed through those regions. Stream flow fills several channels across the floodplain. It was also observed that upstream, the river cuts a narrow canyon into the bedrock, while downstream it broadens and splits as it flows through a floodplain. Upstream, the surface of the bedrock lies above the creek level, and Lynn Creek has only been able to cut a narrow canyon through this hard rock. Downstream, the bedrock surface is buried, and here the river has cut a much broader valley through the softer, thick sediment overlying the bedrock. These sand and gravel sediments are visible in the cliffs at the canyon mouth.

Fig. 1.0: Sketch of the canyon. Within the canyon, the flow of the river runs primarily between narrow walls. This concentrated flow has produced a deep pool in the gravels at the canyon mouth. The scouring of these pools occurs mostly during floods, when the energy of the river is at its highest. As the waters spread downstream, they lose energy, and the pools become generally shallower in the downstream direction.

Evidence of Glaciation in the Canyon

The absence of vegetation along the cliff is a clear indication of active erosion, which is particularly intense during floods when rising water levels lap against the cliff face. During floods, boulders are transported downstream by the fast-moving water, along with gravel and sand likely sourced from as far upstream as the Burrard Inlet. The presence of a boulder layer at the top of the cliff — similar in character to those found within the canyon — indicates an ancient river level from a time when the river flowed at that higher elevation and the flat surface above represented the ancient floodplain, before the river eroded downward to its present level.

Lynn Canyon presents spectacular geographical features, including evidence of major glaciation processes. At the base of the canyon lies a section of the Coast Mountain Range, formed through volcanic activity estimated to have occurred approximately 10 million years ago. Subsequent isostatic adjustment further raised the volcanic range above sea level, completing the orogenic process; an exposed igneous dike serves as evidence of this activity (Geocaching, 2013). This presents a unique combination of numerous geological processes occurring within the same region. Following the formation of the Coast Range, three glacial and inter-glacial periods occurred that eroded the range to its present form.

Lynn Canyon underwent three significant periods of glaciation, the most recent of which — occurring between approximately 20,000 and 10,000 years ago — shaped the canyon to its current state. It was during this period that the U-shaped valley of the canyon was formed, transforming what had previously been large bays with high elevations along the North Shore. This transformation resulted from an ice sheet approximately 2 kilometers thick. One line of evidence for this glaciation is the erratic rocks picked up and deposited along the canyon as the glacier moved toward the Pacific Ocean. Another is the deposits of till and peat located near the suspension bridge (Geocaching, 2013).