Hanging valleys in fluvial systems: Controls on occurrence and implications for landscape evolution

Abstract

[1] We document and characterize hanging valleys in a fluvially eroded landscape in eastern Taiwan. Our conceptual model for the initiation of hanging valleys builds on a recently proposed model of bedrock incision in which erosion actually becomes less efficient on very steep channel gradients. If a pulse of incision in the main stem outpaces the tributary response, the gradients at tributary mouths may therefore pass a threshold value beyond which erosional efficiency declines, giving rise to a mismatch between trunk and tributary erosion rates. This mismatch is expected at junctions with small tributaries, where a step function decrease in drainage area also leads to sharp contrasts in water and sediment flux between trunk and tributary channels. The occurrence of hanging valleys in actively uplifting fluvial landscapes such as the Central Range of Taiwan suggests that the most common parameterizations of bedrock erosion, which typically assume a monotonic positive correlation between channel gradient and incision rate, may be violated in very steep channels. In addition, hanging valleys could greatly increase the response time of landscapes to tectonic perturbations since catchments above these tributary mouths will be insulated from these perturbations until a new suite of processes (e.g., weathering and rock mass failure) wear through the hanging valley lip. The results of this study underscore the need for a more complete understanding of bedrock erosion processes and the incorporation of process transitions and threshold conditions into landscape evolution models.

Characteristics of steady state fluvial topography above fault‐bend folds

“…One prediction of landscape evolution models with sediment-flux-dependent incision is the production of oversteepened reaches [Gasparini et al, 2006;Crosby et al, 2007]. Such steep reaches of streams have been observed to reach slopes up to S % 1 [Schoenbohm et al, 2004;Gasparini et al, 2006;Wobus et al, 2006b;Crosby et al, 2007]. Like hillslopes, the responses of these oversteepened reaches may be affected considerably by lateral advection.…”

“…In the region of bedrock streams, which commonly have slopes up to $0.2, or $11° [Seidl and Dietrich, 1992;Lague and Davy, 2003], the erosional flux of material through a streambed of unit dimensions can be 130% the vertical component (for a reasonable a = 30°). Steeper streams exist and may approach slopes of 1 [e.g., Schoenbohm et al, 2004;Wobus et al, 2006b] highlighting the facts that the dominant erosion mechanism in steep channels is poorly understood but remains an area of active research [e.g., Stock and Dietrich, 2003;Crosby et al, 2007] and the fluvial-colluvial transition is probably much broader than any singular transition slope value implies. In the field of debris flow channels and hillslopes, with gradients up to $1, or $45° [Lague and Davy, 2003;Stock and Dietrich, 2003], the total flux can be 270% the vertical component.…”

Knickzone Extraction Tool (KET) – A new ArcGIS toolset for automatic extraction of knickzones from a DEM based on multi-scale stream gradients

“…[18][19][20]. In spite of the widening scope of research in knickzone modeling [21], generating a regional scale map of knickzones remains a challenge.…”

How concave are river channels?

“…One complication of using collinearity to calculate the most likely m/n value is that occasionally one may find a hanging 5 tributary (e.g., Wobus et al, 2006b;Crosby et al, 2007), which could occur for a variety of reasons, such as the presence of geologic structures or lithologic variability. A hanging tributary can skew the overall MLE values in a basin, so in each basin we test the MLE and RMSE values in each tributary for outliers and iteratively remove these outlying tributaries, testing for the most likely m/n value on each iteration.…”

“…Additionally, these catchments may in effect behave as fluvial hanging valleys (Wobus et al, 2006b). Values of m/n derived using the Monte Carlo points method are in all cases equal to or lower than values derived using all χ data.…”