Tsuyoshi Hattanji

The Climatic Signature of Incised River Meanders

Messy Mountain Meandering Predicting the influence of climate on landscapes is sometimes straightforward; for example, river deposits might grow with increased rainfall because erosion rates and sediment transport increase. However, long-term tectonic processes complicate the geomorphic signatures of more gradual climate-related phenomena that reconfigure landscapes. By correlating a decades-long record of typhoon rainfall in Japan with digital elevation models, Stark et al. (p. 1497) show that climate directly influences the extent of river meandering. When expanded to a larger region of the western North Pacific, this analysis revealed a strong climatic imprint on the landscape of humid mountainous areas. The region-wide analysis also revealed that underlying bedrock strength, as opposed to tectonic uplift, acts as a secondary control. Typhoon frequency and bedrock strength influence river meandering in mountain environments. Climate controls landscape evolution, but quantitative signatures of climatic drivers have yet to be found in topography on a broad scale. Here we describe how a topographic signature of typhoon rainfall is recorded in the meandering of incising mountain rivers in the western North Pacific. Spatially averaged river sinuosity generated from digital elevation data peaks in the typhoon-dominated subtropics, where extreme rainfall and flood events are common, and decreases toward the equatorial tropics and mid-latitudes, where such extremes are rare. Once climatic trends are removed, the primary control on sinuosity is rock weakness. Our results indicate that the weakness of bedrock channel walls and their weakening by heavy rainfall together modulate rates of meander propagation and sinuosity development in incising rivers.

High-Resolution Analysis of Debris Flow–Induced Channel Changes in a Headwater Stream, Ashio Mountains, Japan∗

Coupled hillslope and channel processes in headwater streams (HWS) lead to rapid changes in channel dimensions. Changes in channel size and shape caused by a debris flow event along the length of a headwater stream in the Ashio Mountains, Japan, were captured with the aid of repeat high-definition surveys using terrestrial laser scanning (TLS) techniques. The HWS was classified into three distinct reaches below the debris flow initiation zone. A large knickpoint separated an upper bedrock reach from a colluvial reach along the midsection of the drainage. The colluvial reach transitioned to a lower bedrock reach that terminated at the master stream. Cross-sectional and morphometric analyses revealed no statistically significant changes in channel size or shape along the upper bedrock reach. Debris flow erosion generated significant differences in channel size and shape along a colluvial reach. Sediment bulking associated with erosion along the colluvial reach led to increases in channel size along the lower bedrock reach, but no statistical differences in channel shape. Morphometric analyses from the TLS point cloud revealed that debris flow erosion produced a distinct nonlinear change in channel dimensions in the downstream direction within the HWS. Variations in channel substrate along the length of HWS contributed directly to this nonlinear response. The episodic nature and nonlinearity of erosion associated with the current debris flow event highlights the importance of debris flows in general in understanding the transport of sediment, coarse to fine particulate organic material, and large woody debris, which are critical to the long-term management of riverine environments. TLS sampling methods show promise as one component of a multianalytical approach needed to continuously monitor and manage the dynamics of HWS.

Evolution of solution dolines inferred from cosmogenic 36Cl in calcite

Quantification of the development of solution dolines provides important information for understanding the long-term evolution of karst landscapes. This study reports the initial results of an investigation of the long-term denudation rates along the side slopes of a solution doline based on analyses of cosmogenic 36 Cl in calcite. The denudation rates increase in proportion with increasing size of the topographic contributing area, thereby supporting the hypothesis that the rate of surface lowering in carbonate terrains is controlled by water convergence in the epikarst. A simple model based on the empirical correlation between denudation rate and contributing area is successful in explaining the form of several solution dolines located close to the analyzed doline. The model reveals that these solution dolines, which have varying diameters, developed over similar time scales of the order of 10 5 yr.

Hydrogeomorphic Effects of Basin Lithology on Development of Channel Steps in First-Order Basins of the Ashio Mountains, Japan

This study focuses on lithological contrasts of headmost channel morphology of two mountainous areas underlain by chert or sandstone. The areas have contrasting runoff processes: subsurface storm flow predominates in the chert area, whereas groundwater flow recharges stable stream flow in the sandstone area. Manual field measurements from 84 sections of first-order streams revealed that channel steps develop along 26% of all sections in the chert area, while only along one section (3%) in the sandstone area. High-resolution analysis of long profiles using terrestrial laser scanning detected a weaker pattern of channel steps in a colluvial reach of the sandstone basin. The contrast in runoff processes, as well as grain-size distributions of weathering products between the two areas, influences dissimilar bed-load transport regimes and promotes spatial variations in channel profiles between headwater streams. These contrasting hydro-geomorphic processes affect channel-step morphology, indicating that lithology plays a key role in the formation of channel steps and in variations between headwater streams.