Controls on anastomosis in lowland river systems: Towards process-based solutions to habitat conservation

Abstract

Anastomosing rivers were historically common around the world before extensive agricultural and industrial development in river valleys. Few lowland anastomosing rivers remain in temperate zones, and the protection of these river-floodplain systems is an international conservation priority. However, the mechanisms that drive the creation and maintenance of multiple channels, i.e. anabranches, are not well understood, particularly for lowland rivers, making it challenging to identify effective management strategies. This study uses a novel multi-scale, process-based hydro-geomorphological approach to investigate the natural and anthropogenic controls on anastomosis in lowland river reaches. Using a wide range of data (hydrologic, cartographic, remote-sensing, historical), the study (i) quantifies changes in the planform of the River Narew, Poland over the last 100years, (ii) documents changes in the natural and anthropogenic factors that could be driving the geomorphic change, and (iii) develops a conceptual model of the controls of anastomosis. The results show that 110km of anabranches have been lost from the Narew National Park (6810ha), a 42% reduction in total anabranch length since 1900. The rates of anabranch loss have increased as the number of pressures inhibiting anabranch creation and maintenance has multiplied. The cessation of localized water level and channel management (fishing dams, water mills and timber rafting), the loss of traditional floodplain activities (seasonal mowing) and infrastructure construction (embanked roads and an upstream dam) are contributing to low water levels and flows, the deposition of sediment at anabranch inlets, the encroachment of common reed (Phragmites australis), and the eventual loss of anabranches. By identifying the processes driving the loss of anabranches, this study provides transferable insights into the controls of anastomosis in lowland rivers and the management solutions needed to preserve the unique anastomosing river pattern and diverse wet grasslands that are central to the conservation value of lowland floodplains.

Long-Term Downstream Effects of a Dam on a Lowland River Flow Regime: Case Study of the Upper Narew

“…Frequent or high-magnitude flooding is assumed as a precondition for avulsion and the eventual formation of new channels. The SR-induced decrease of the magnitude and duration of floods can, therefore, be deemed as one of the factors, among others recognized by Marcinkowski et al [23], speeding up the gradual extinction of the valuable anastomosing character of the Narew River. It is critical to address this issue in any amendments to contemporary water management instructions governing the flow regime of the Narew River.…”

“…Although the influence of the SR operation on the average and median flows is not significant compared with studies of Mioduszewski et al [27] and Cygan et al [24], and as presented by Kiczko et al [26]-strongly depends on the river stretch, we revealed the influence of the SR on the extreme flow of the Narew River. Marcinkowski et al [23] investigated seasonal changes in water flow and indicated that average flows are lower in April but higher in February and March, which is determined by dam operations, in which water is released from the reservoir in advance of the spring thaw to prevent flooding in municipalities close to the reservoir (see Supplementary material, Section 2).…”

“…The NNP, located downstream of the SR, has been facing the loss of anabranches in recent decades [23]. It has been recognized that the highly variable flood-prone flow regime characterized by the occurrence of seasonal high discharges is one of the most crucial factors for global anastomosing rivers to persist [47,48].…”

Modeling of water flow in multi-channel river system in the Narew National Park

“…The river valley is occupied by wetlands with early growth sedge and reed communities. In recent decades uncontrolled expansion of common reed (Phragmites australis) became a serious problem causing signifi cant loss of biodiversity (Próchnicki 2005) and the loss of anabranches (Marcinkowski et al 2017). The abundance of aquatic vegetation in river channels depends on the water depth and current velocity.…”

“…They were historically common before extensive agricultural and industrial development in river valleys (Walter and Merritts 2008, Lewin 2010, Marcinkowski et al 2017. Presently, few of them persist and their protection is an international conservation priority.…”