Will Honeymoon Bay Ecological Reserve Still Have Erythroniums 50 years from now?
From: THE LOG • FRIENDS OF ECOLOGICAL RESERVES NEWSLETTER • AUTUMN 2005
by David F. Polster, M.Sc., R.P.Bio. Plant Ecologist
Construction of the logging road bridge across Sutton Creek at the upstream edge of the ecological reserve has caused a significant change in the hydrology of the creek and consequently the ecology of the reserve. Prior to bridge construction the creek would flood over the banks on an annual basis. This deposited a thin layer of fresh sediment on the floor of the reserve, burying seedling that were starting to establish, and more importantly, washing away the seeds of the big leaf maple. The pink fawn lilies and other perennial species were able to push up through this thin sediment layer, but many of the species that are now seen dominating the lily areas such as the false bugbane would have been suppressed. More importantly, the maple seedlings that are now taking over the reserve and will eventually suppress the lilies would not establish during the regime of flooding except in rare circumstances thus we would see old maples with no young ones in the understory.
So how is it that the bridge has caused this shift in the natural disturbance regime that is essential for the maintenance of the lily populations, and what can be done about it? When the bridge was built, rather than providing a bridge built on timber bents (groups of pilings set in a line perpendicular to the bridge that are used to support the bridge), large approach fills were constructed and a free span bridge was installed. However, as the creek flowed over a floodplain that is several hundred meters wide, the construction of the approach fills has resulted in constricting the creek during high flow periods (typically November and December just after the maple seeds have been deposited on the forest floor). The constriction in the creek creates an increase in the flow velocity that in turn causes scouring of the stream bed. The streambed in the vicinity of the bridge has dropped approximately one meter due to the loss of bed materials. This depression in the bed elevation extends for several hundred meters upstream and downstream of the bridge. The materials that have been lost from the bed are being deposited in large gravel bars downstream, resulting in bank erosion and other signs of hydrologic distress.
Remedial work would entail creating a porous bridge structure (pile supported?) and raising the bed elevation. Newbury riffles could be constructed to raise the bed elevation while enhancing the fisheries values of this section of stream in terms of spawning habitat. Detailed hydrologic studies would be required to determine appropriate bridge design as well as channel design. Given the extent of maple in-growth in the reserve, remedial work would also entail cutting the maple saplings that now cover the floodplain. Monitoring would be needed once annual flooding was re-established to see if restoring the natural disturbance regime would serve to reverse the ecological processes associated with elimination of flooding. It is not likely that BC Parks, due to limited budget, will have the resources to carry out this work. ■