Stuart Island geology


locator mapThe point of a map is to help us get where we're trying to go. In this case, knowing the underwater geography of a place can help us choose where to focus our investigations and conservation efforts.

For example, this map helps us identify good habitat for rockfish, one of the major conservation targets in the Salish Sea. Several rockfish species are protected under the US Endangered Species Act. All species are closed to fishing on the US side. British Columbia has set up 164 Rockfish Conservation Areas where no hook and line fishing is allowed.

But one of the biggest things this map shows is that we have more work to do. The shallow areas around Stuart and Johns Islands have not been mapped. (Shallow-water work involves a different boat and greater caution than the deep-water mapping.) We expect to find more rough rocky habitat around the edges and more even sediments in the embayments, but we won't really know what's there until we map it.

What else to see on this map:

1. Numerous faults slash through the area. Visible on this map are fault scarps running approximately NW to SE along the top of Stuart and Johns Islands. Two scarp locations are marked on the map and are probably part of the same fault, but they only show up on the bathymetry in these two places. There’s another scarp near the 8 on the distance scale at the bottom of the image, and yet another striking in a WNW direction from the bottom right of the map.

As if that wasn’t evidence enough of past tectonic activity, there’s also the 3.9 earthquake that happened on December 26, 2013. The epicenter of this quake is shown on the map, though it was several kilometers down and there’s no visible fault seen in the bathymetry.

2. Evidence of past glaciers includes two possible moraines where the retreating glaciers might have paused. The Haro Strait / Boundary Pass channel is a glacial fjord carved by glaciers moving down from the north. We can see two places that might be remnants of moraines where the glaciers paused in their retreat or melting episode and dumped a load of sediment concentrated along their leading edge. The southwest one is fairly small, but the one between South Pender Island and Stuart Island is much longer and more substantial in character.

dfo currents3. Scouring from strong currents. If you’re water heading north or south through this area, chances are you race through Haro Strait and Boundary Pass.  All this water force has scoured out the bottom of the channel. You can even see a circular depression NW of Turn Point where turbulence has created a scour hole. Between the scour hole and Turn Point, look for the rugged projection of the island underwater. This is a tough chunk of sedimentary rock that withstood the glacier and continues to hold its own against the currents.

In another view on this same map, below, you can see different potential habitat types identified. Typical rockfish habitat is marked with dotted horizontal lines. The areas with wavy lines are “dynamic bedforms” consisting of sand, and are likely good habitat for sand lance, an important forage fish species.

stuart habitat

Maps by G. Greene & J. Aschoff.