José Antonio Constantine

Meander cutoff and the controls on the production of oxbow lakes

Using satellite images archived by Google Earth™, we measured channel and oxbow-lake characteristics of 30 large meandering rivers to identify the controls on the production of oxbow lakes by meander cutoff. Cutoff produced lognormal distributions of lake lengths within the studied reaches, and the geometric mean lake length of each population correlated positively and exponentially with sinuosity, due to more highly sinuous reaches being comprised of longer meanders and to cutoff removing longer segments of more sinuous channels. We successfully predicted the size-frequency distributions of lakes stored within the flood-plains of five freely meandering reaches using only channel sinuosity and an assumption of the variance about the geometric mean lake length, a variable that did not significantly vary between the studied reaches. While the rivers sinuosity remains steady, the temporal rate of cutoff can be estimated using channel sinuosity, the fraction by which cutoff reduces channel length, and the rate at which the reach lengthens by meander growth.

A mechanism of chute cutoff along large meandering rivers with uniform floodplain topography

Incidents of chute cutoff are pervasive along many meandering rivers worldwide, but the process is seldom incorporated into theoretical analyses of planform evolution, partly due to the paucity of observations describing its physical controls. Here, we describe a mechanism of chute cutoff that may be prevalent along large meandering rivers with uniform floodplain topography. The mechanism occurs independently of sudden changes in conveyance capacity, such as those caused by natural dams, and instead, it is initiated during a flood by the incision of an embayment. The embayment is typically located almost a channel width upstream of the entrance to the meander that undergoes cutoff, and subsequent floods extend the embayment downstream until a chute is formed. Using sequences of historical aerial photos of the Sacramento River in California, USA, we found that embayments formed where channel curvature was greatest, or where the channel most tightly curved away from the downstream flow path. Embayments formed only within those portions of the floodplain that were lightly vegetated by grasses or crops. We develop a simple physical model that describes the environmental conditions that can lead to embayment formation. The model considers the role of floodplain vegetation in preventing chute incision and in part explains why chute cutoff is prevalent along some meandering rivers but not others.

Generic theory for channel sinuosity

Sinuous patterns traced by fluid flows are a ubiquitous feature of physical landscapes on Earth, Mars, the volcanic floodplains of the Moon and Venus, and other planetary bodies. Typically discussed as a consequence of migration processes in meandering rivers, sinuosity is also expressed in channel types that show little or no indication of meandering. Sinuosity is sometimes described as “inherited” from a preexisting morphology, which still does not explain where the inherited sinuosity came from. For a phenomenon so universal as sinuosity, existing models of channelized flows do not explain the occurrence of sinuosity in the full variety of settings in which it manifests, or how sinuosity may originate. Here we present a generic theory for sinuous flow patterns in landscapes. Using observations from nature and a numerical model of flow routing, we propose that flow resistance (representing landscape roughness attributable to topography or vegetation density) relative to surface slope exerts a fundamental control on channel sinuosity that is effectively independent of internal flow dynamics. Resistance-dominated surfaces produce channels with higher sinuosity than those of slope-dominated surfaces because increased resistance impedes downslope flow. Not limited to rivers, the hypothesis we explore pertains to sinuosity as a geomorphic pattern. The explanation we propose is inclusive enough to account for a wide variety of sinuous channel types in nature, and can serve as an analytical tool for determining the sinuosity a landscape might support.

Modification of river meandering by tropical deforestation

Tropical forests are the only forest biome to have experienced increased rates of forest loss during the past decade because of global demands for food and biofuels. The implications of such extensive forest clearing on the dynamics of tropical river systems remain relatively unknown, despite significant progress in our understanding of the role of trees in riverbank stability. Here, we document rates of deforestation and corresponding average annual rates of riverbank erosion along the freely meandering Kinabatangan River in Sabah, Malaysia, from Landsat satellite imagery spanning A.D. 1989–2014. We estimate that deforestation removed over half of the river’s floodplain forest and up to 30% of its riparian cover, which increased rates of riverbank erosion by >23% within our study reaches. Further, the correlation between the magnitude of planform curvature and rates of riverbank erosion only became strongly positive and significant following deforestation, suggesting an important role of forests in the evolution of meandering rivers, even when riverbank heights exceed the depth of root penetration.

Colluvium supply in humid regions limits the frequency of storm-triggered landslides

Shallow landslides, triggered by extreme rainfall, are a significant hazard in mountainous landscapes. The hazard posed by shallow landslides depends on the availability and strength of colluvial material in landslide source areas and the frequency and intensity of extreme rainfall events. Here we investigate how the time taken to accumulate colluvium affects landslide triggering rate in the Southern Appalachian Mountains, USA and how this may affect future landslide hazards. We calculated the failure potential of 283 hollows by comparing colluvium depths to the minimum (critical) soil depth required for landslide initiation in each hollow. Our data show that most hollow soil depths are close to their critical depth, with 62% of hollows having soils that are too thin to fail. Our results, supported by numerical modeling, reveal that landslide frequency in many humid landscapes may be insensitive to projected changes in the frequency of intense rainfall events.