TY - JOUR
T1 - Timescales of tree-covered island dynamics on the mixed bedrock-alluvial anabranching Vaal River, South Africa
AU - Tooth, S.
AU - Keen-Zebert, A.
AU - Grenfell, M. C.
AU - Addison, G.
N1 - Publisher Copyright:
© 2024 The Authors. River Research and Applications published by John Wiley & Sons Ltd.
PY - 2024/7/10
Y1 - 2024/7/10
N2 - Previous research on mixed bedrock-alluvial anabranching rivers has documented how alluvial islands commonly grow under vegetation influences atop slowly eroding bedrock templates, but timescales of island dynamics remain poorly constrained. We focus on the Vaal River near Parys, South Africa, and combine field investigations, aerial image analyses and optically stimulated luminescence (OSL) dating to establish timescales of initiation, growth and erosion for nine bedrock-cored, tree-covered, alluvial islands. For each island, two OSL samples were collected in vertical succession from sand-rich exposures up to 4 m in thickness to establish minimum ages for island initiation (~1802 to 243 years) and to estimate local vertical aggradation rates (~0.20 to 1.8 cm year−1). The diachronous lower ages and lack of systematic upstream-downstream trend in island age support an interpretation of patchwork initiation, growth, and erosion of islands throughout the late Holocene. Following island initiation, vertical island aggradation occurs in association with establishment of reeds, shrubs and trees, but erosion of island margins or dissection by cross-cutting channels also can occur. Observations during and after recent large floods (peak discharges >3000 m3 s−1) provide further insights into island dynamics, including the influence of exotic trees (e.g., Eucalyptus spp.) that have colonised many islands in the postcolonial era (last ~150 years). Our findings extend previous conceptual models by constraining timescales of island dynamics and providing new insights into island stability and longevity in mixed bedrock-alluvial anabranching rivers. Improved communication of findings regarding island geomorphology, ecology and stability can benefit local community engagement, geo/eco-tourism and education activities, and land use planning.
AB - Previous research on mixed bedrock-alluvial anabranching rivers has documented how alluvial islands commonly grow under vegetation influences atop slowly eroding bedrock templates, but timescales of island dynamics remain poorly constrained. We focus on the Vaal River near Parys, South Africa, and combine field investigations, aerial image analyses and optically stimulated luminescence (OSL) dating to establish timescales of initiation, growth and erosion for nine bedrock-cored, tree-covered, alluvial islands. For each island, two OSL samples were collected in vertical succession from sand-rich exposures up to 4 m in thickness to establish minimum ages for island initiation (~1802 to 243 years) and to estimate local vertical aggradation rates (~0.20 to 1.8 cm year−1). The diachronous lower ages and lack of systematic upstream-downstream trend in island age support an interpretation of patchwork initiation, growth, and erosion of islands throughout the late Holocene. Following island initiation, vertical island aggradation occurs in association with establishment of reeds, shrubs and trees, but erosion of island margins or dissection by cross-cutting channels also can occur. Observations during and after recent large floods (peak discharges >3000 m3 s−1) provide further insights into island dynamics, including the influence of exotic trees (e.g., Eucalyptus spp.) that have colonised many islands in the postcolonial era (last ~150 years). Our findings extend previous conceptual models by constraining timescales of island dynamics and providing new insights into island stability and longevity in mixed bedrock-alluvial anabranching rivers. Improved communication of findings regarding island geomorphology, ecology and stability can benefit local community engagement, geo/eco-tourism and education activities, and land use planning.
KW - flood
KW - luminescence dating
KW - biogeomorphology
KW - riparian vegetation
KW - Holocene
KW - mixed bedrock‐alluvial river
UR - http://www.scopus.com/inward/record.url?scp=85192251143&partnerID=8YFLogxK
U2 - 10.1002/rra.4296
DO - 10.1002/rra.4296
M3 - Article
AN - SCOPUS:85192251143
SN - 1535-1459
VL - 40
SP - 1049
EP - 1066
JO - River Research and Applications
JF - River Research and Applications
IS - 6
ER -