Identification of Criteria for Determining the Restoration Potential of Riparian Wetlands in Highly Degraded Agricultural Areas

Riparian wetlands result from the lateral migration and cutoff of river meanders. Abandoned meander channels, or oxbow lakes, form riparian wetlands rich in biodiversity that remain hydrologically connected to the waterway.

In agricultural areas, several thousand kilometres of winding waterways were straightened and their abandoned meander channels backfilled during the 20th century, resulting in a significant loss of riparian wetland area in addition to the destruction of the process for the formation of new riparian wetlands.

The rehabilitation or reconnection potential of old meander channels, allowing for the future creation of riparian wetlands in Québec, could serve to guide measures for compensation and adaptation to climate change at the regional level.

• Identify old meander channels or depressions in fields close to waterways with the highest potential for wetland and ecosystem services restoration.

• Analyze projects to rehabilitate and reconnect abandoned meander channels in other parts of the world in order to identify key restoration indicators and benefits.

• Study the hydrological connection between the three straightened waterways under study and their old meander channels (morphological and hydrological measures).

• Model under current and future climate scenarios the potential for remeandering and the creation of new riparian wetlands on the three sites.

• Perform a combined analysis of the literature review and the modelling results to propose criteria for the restoration potential of riparian wetlands in agricultural areas.

Through a review of the knowledge on the restoration of old meanders, indicators of project success or failure were identified in the literature ahead of a detailed study of the restoration potential of the wetlands of three straightened watercourses in the St. Lawrence Lowlands in Quebec.

The three watercourses studied (about 1 km long) are located in intensively farmed regions (mainly corn and soybeans) on the Des Fèves River, on the Petite Rivière Pot-au-Beurre and on the Ruisseau Martin stream. All three sections are located in the clayey loam geological environment of the Champlain Sea, partially eroded by streams and accompanied by coarser sediments in the alluvial plain.
At each site, the physical characteristics of the channel and the old meander (channel geometry, meander topography, surface geology, water level at gauging stations as well as in wells, piezometers, temperature) and ecological components (riparian vegetation and fish) were measured by a combination of quantitative methods (e.g. pressure sensors) and qualitative methods (e.g. interpretation by experts and cameras).

The potential for hydrological connections through surface and/or underground processes was assessed using signal processing techniques to determine times between maximum water levels in the channel and in piezometers in response to significant precipitation. The potential for re-meandering was assessed using the RVR Meander model to simulate channel migration.
Clear differences between the sites were found in terms of both restoration potential and hydrological connectivity. The cohesion of the clay banks at the Petite Rivière Pot-au-Beurre and the Des Fèves River probably limits the migration of the channel at these sites. Persistent human activities such as dredging, removal of riparian vegetation and filling of the old meander with thick layers of fine materials have led to the disconnection of the coarse alluvial deposits from the channel, with no sign of lateral hydrological connectivity at these sites.

At the Ruisseau Martin, many factors, such as the change in land use from agriculture to forestry following the initial straightening (last straightening between 1950 and 1960), have resulted in significant potential for lateral hydrological connectivity and the passive restoration of the riparian wetlands (Figure 1).

Figure 1. Simulation of the re-meandering potential of the Ruisseau Martin (A. upstream section B. downstream section) with the RVR Meander model in the current climate (2014) and in the projected climate for the next 100 years based on different passive or active restoration assumptions: no change (sim actual parameters), decrease of the slope of the bank by 25% (sim decreased slope),  increase in flow in proportion to the increase in total annual precipitation projected by 12 climate change scenarios (sim increased discharge) and widening of the channel by 25% (sim increased width).

The results of the vegetation and fish habitat assessments showed that the Ruisseau Martin had the highest proportion of its floodplain classified as wetlands and an excellent quality of fish habitat in the channel sections adjacent to the old meanders.

The active restoration approach will improve the topography of the old meanders and the width of the riparian strip between the old meander and adjacent agricultural fields, thereby promoting channel migration and restoring natural hydrogeological processes in order to create wetlands that are dynamic in time and space.
dans le temps et dans l'espace.

Contribution d'Ouranos dans ce projet sous forme d'expertise

• Université du Québec à Rimouski

• Université du Québec à Montréal

• Pleine-Terre

• Institut de recherche et de développement en agroenvironnement