Bank Armour and Riparian Disturbance in the Bow River Basin

Bank Armour and Riparian Disturbance in the Bow River Basin

 

Following flooding in the Bow River basin in 2005, Trout Unlimited Canada became increasingly concerned about the amount of bank armouring (both authorized and unauthorized) along rivers within the watershed. Much of this involves riprap or “hard armouring” of river banks to protect property and infrastructure from river bank erosion. As increasingly extensive portions of rivers become armoured, they lose functionality from a lack of connection to their floodplains, and these degraded riparian areas fundamentally alter natural river processes that

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Riprap along the upper Elbow River (photo credit East Slopes Aquatic Consulting)

maintain natural channels and healthy fish habitat. In 2006, Trout Unlimited Canada commissioned an inventory of bank armouring along the Elbow, Sheep, and Highwood Rivers and documented 30, 40, and 52 disturbances, respectively (Popowich and Eisler 2007). Following flooding in 2013, we wanted to know if and how this had changed. Although much of the bank armouring completed following the 2013 flood was authorized, there is no comprehensive inventory of the extent of bank armouring and riparian disturbance along these rivers.

In 2016, TUC hired East Slopes Aquatic Consulting to replicate the inventory completed in 2006. Over 300 km of river were traversed by boat and each riparian disturbance was catalogued and photographed. The Lower Elbow River and reaches of the Bow River upstream and downstream of Calgary were also surveyed and Threepoint Creek was assessed using satellite imagery. A total of 317 locations of riparian disturbances were encountered during the field survey. The results confirmed an increase in the frequency and cumulative length of riparian disturbances in the upper Elbow River, Sheep River, and Highwood River reaches (those that were also surveyed in 2006).

Generally, most of the riparian disturbances consisted of hard armouring, and was often found in proximity to where private and public infrastructure is located near active channels or within floodplains (e.g. in towns and cities, near railway tracks or bridges). the Lower Elbow River had the greatest proportion of disturbance and was the only reach located entirely within an urban setting.

Not all of the riparian disturbances consisted of riprap. Impacts from cattle were also included as well as other access points (e.g. boat launches, water intakes, etc.). In some cases, “softer

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Riprap along the Sheep river (photo credit East Slopes Aquatic Consulting)

” approaches to bank armouring were utilized with varying levels of success. This includes the use of bioengineering (using live plant material to provide some engineering function) and/or incorporating large woody debris into structures to provide some ecological function. These techniques can actually improve the health of the site over time as vegetation becomes established and can be quite cost effective.

Several sites were identified where some degree of armouring failure had occurred since 2006. The crew also noted several of the sites that were documented as disturbed due to livestock impacts in 2006 have since recovered, demonstrating the resiliency of watersheds if given the opportunity to recover. Some restoration opportunities were also identified where bioengineering techniques could be used to restore riparian health and function.

The inventory results should serve as a reminder that we would not have to armour river banks if we stayed out of the way of the river in the first place. To make our watersheds more resilient, we should not be thinking about how to prevent flooding, but how to allow natural processes to occur without significant damage to human infrastructure.

Thank you to the following for providing financial or in-kind support to this project: Alberta Environment and Parks and the Watershed Resiliency and Restoration Program, Alberta Conservation Association, East Slopes Aquatic Consulting, Hemmera Envirochem Inc. and Golder Associates Ltd.

A copy of the report is available here.