Portage Inlet has undergone significant changes over the millennia. The area was once dry land, probably before the most recent glacial period, the Fraser Glaciation 29,000 to 13,000 years ago. When the ice sheets began to melt, sea level rose quite rapidly and the ocean flooded the Gorge and Portage Inlet. This is evident from deposits of marine clay found 2-3 meters below the present sea level.
Following glaciation, Vancouver Island gradually rose from the sea as it rebounded from under the weight of ice. Consequently, for a time, between about 9,250 and 5,470 years ago, Portage Inlet was cut off from the sea again. Geological excavations have revealed a layer of peat littered with pine cones (Pinus contorta
and Pinus monicola
). This indicates that the area was a swamp surrounded by forest.
Eventually, the rebound of the land slowed, while the sea level around the world continued to rise due to melting glaciers. By about 5,500 years ago the sea level had stabilized at its present height, and the ocean flooded back into Portage Inlet. A one meter thick deposit of marine clay containing Olympia oyster (Ostrea lurida
) shells provides evidence of this most recent event.
- Today, the shoreline of Portage Inlet is largely altered, as many areas have been filled in and seawalls have been built. Exceptions include rocky shorelines along Christie Point and along the southeast shore.
- The water depth in Portage Inlet is generally less than two meters; this results in water temperatures that can reach 20°C during the summer.
- Tidal flushing is limited, due to the narrow connection via the Gorge to the outside harbours, and there is a four hour time lag between the change in tide in the outer harbour areas and in Portage Inlet. During the summer, when stream flows are low and waters are more stagnant, evaporation can be significant in Portage Inlet, resulting in increased salinity levels. During the winter, more fresh water flows from the streams and accumulates in the inlet, and salinity can reach low levels. This explains why the water in Portage Inlet sometimes freezes in the winter, unlike most ocean water in this area, since salty water freezes at a lower temperature. These factors influence the marine life in the inlet. Limited tidal flushing can also cause pollutants and sediment to accumulate in the inlet.
- Sediment, in the form of mud, sand and gravel, covers the bottom of the inlet. Mud is the dominant sediment type, covering 75% of the bottom. Sediment has been accumulating in Portage Inlet for thousands of years. However, some people are concerned that erosion in upland areas is contributing to excessive sedimentation that could eventually fill in the inlet.
- A large intertidal mud flat is located at the mouth of Craigflower Creek on the west side of the inlet, and a smaller one is found at the mouth of Colquitz Creek on the east side of the inlet.
- Chris J. Yorath, 2005. Geology of Southern Vancouver Island. Harbour Publishing, Revised Ed.