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Author(s): Simon J. Dadson (corresponding author) [1]; Niels Hovius [1]; Hongey Chen [2]; W. Brian Dade [4]; Meng-Long Hsieh [2]; Sean D. Willett [5]; Jyr-Ching Hu [2]; Ming-Jame Horng [6]; [...]

Fluvial bedrock incision, which creates topographic relief and controls hillslope development, has been considered the key medium linking denudation and tectonic uplift of unglaciated mountains. This article, however, shows a different scenario from the Coastal Range in eastern Taiwan. This range, with the steepness inherited from pre-orogenic volcanoes, has been subject to mass wasting even before its emergence above sea level no earlier than Middle Pleistocene. Numerous terraced alluvial fans/fan deltas record the ancient mass movements of the range, including rock avalanches. Multiple radiocarbon dates <16 ka cal BP reveal the recurrence intervals of these movements of over several thousand years. The largest event is dated ∼15 ka cal BP, and the two second largest, 9–8 ka cal BP. These mass movements were sourced from ridges with minimum heights of 350–400 m, have sequences not clearly related to the known climate-change events, and are believed to have been triggered mainly by severe rainfall events, large earthquakes, or their combinations. The resulting fluctuation of sediment yield has episodically changed river behavior, forming river terraces in catchments >1 km 2 . Alluvial terraces are typically exhibited close to the source ridges of mass movements, and strath terraces along the downstream parts of rivers. Both were created when enormous sediment supply had exceeded or matched the prevailing river transport capacity. This process, along with the protection by giant boulders from mass movement, disturbed the long-term incision trend of rivers in response to tectonic uplift. As a result, the observed Holocene bedrock incision at most sites has not kept pace with the tectonic uplift. The spatial contrast in mass-wasting histories further accounts for the great diversity of the terrace sequences, even in areas with similar tectonic and base-level conditions. [ABSTRACT FROM AUTHOR]

Abstract: Although dominated by erosion over long term, the tectonically active mountains of Taiwan commonly contain thick landslide and debris-flow gravels capping hillslopes or forming alluvial terraces. These deposits and their associated landforms serve to study ancient mass-wasting histories and their controls on fluvial processes. This study focuses on the Pa-chang River draining the <2000m high Ali-shan area in southwestern Taiwan (current tectonic uplift: 5–8mmyr−1). Although small (catchment area: 83km2), the Pa-chang exhibits one of the largest alluvial terraces in Taiwan, which stretches from the outlet of the catchment for 9km along the river and comprises fluvial and debris-flow gravels up to 150m thick. Totally 55 radiocarbon dates have been obtained, tracing the mass-wasting records back to 14.8ka. The largest events, which created the studied major terraces, occurred in 10.0−11.3 and 8.7−9.4ka. These events are synchronous with the formation of some regionally largest alluvial terraces in Taiwan, and may have been triggered by the frequent heavy rains associated with the early Holocene East Asian summer-monsoon maximum. More mass-wasting events have been dated at ~5.4, ~1.9, 1.2−1.7, 0.7−0.9, ~0.6, and <0.3ka. The vast supply of sediment from the recorded mass-wasting events not only caused river aggradation but also regulated the subsequent incision by forming boulder armors. As a result, the bedrock incision in the upper part of the catchment has been negligible since the early Holocene debris-flow deposition. The incision rates increase to the downstream direction as the size and concentration of boulders reduce. Still, the 14.8ka-averaged bedrock incision rate (<4.5mmyr−1) at the catchment outlet is lower than the current uplift rate (6mmyr−1). At the same site, however, the bedrock incision rate is 23mmyr−1 averaged over the past 0.6ka. Even higher rates of incision (>50mmyr−1) are detected further downstream after the recent gravel mining. The fact that the obtained bedrock incision rates and patterns are decoupled with the tectonic uplift advises caution in using short-term erosion data to interpret the tectonics of the Taiwan's mountains. [Copyright &y& Elsevier]

Abstract: With cumulative rainfall of 1700mm, Typhoon Morakot triggered a catastrophic landslide at Hsiao-lin (9 August, 2009), which dammed the river, buried the village and killed>400 people. We conducted a geomorphic study to understand the origin of this landslide in the context of the long-term evolution of the hillslope. The landslide originated from a broad, poorly drained slope (21°–23°) and ran down along two first-order channels. The erosion, up to 80m thick, has created a concave-shape slope and exposed mostly loose debris, which implies that the landslide was mainly the reworking of ancient colluviums. The genesis of the concave-shape slope where the source sediments of the landslide resided reflects the weakness of the mudstone/shale bedrock underlying this slope, in contrast to the sandstone-based transport area. The two channels in the transport area also follow old faults or joints. Given that the source area has been subject to erosion during large landslides, the subsequent deposition here was likely achieved by minor colluvial processes. This deposition continued until the catastrophic failure in 2009, which could be controlled by (1) the amount of sediments deposited, which determined the slope angle, and (2) the headward expansion of the channels from the transport area, which enhanced groundwater convergence and later provided routes for the landslide materials to run downslope. Extensive mass-wasting motions had occurred in the area, as shown by the prevalence of >20m thick landslide/debris-flow sequences on the lower part of the hillslope, which are dated 21, 14.9, 13.7 and 12.0ka by the radiocarbon method. These mass-wasting events had hindered the long-term incision of the trunk river in response to the rapid tectonic uplift. At least the 12.0ka event, which caused extensive mud-flow deposition, had dammed the river. Overall, this study shows how bedrock lithology and structures controlled the shape of a hillslope and caused extremely unsteady mass-wasting transport in the active mountains of Taiwan. [Copyright &y& Elsevier]

Abstract: Landslides and debris flows rarely occurred during historical times in the tectonically active Coastal Range of eastern Taiwan. This topographic stability, however, contrasts greatly with the widespread existence of terraced alluvial fans and fan-deltas on the Hua-tung coast which fringes the range. This study focuses on the two largest fan–terrace systems on the Hua-tung coast, both of which consist of alluvial fans (plane-view areas up to 8km2) larger than their contributing catchments. Stratigraphic data show that both systems were in sandy, wave-dominated settings during de-glacial times. The systems were then disturbed by a catastrophic landslide/debris-flow event (or events), which brought enormous amounts of gravel (Facies Gm) into the systems, deforming previously-deposited marine sands (Facies Sm) and shallowing the seafloor. The combined Gm/Sm complex yields multiple radiocarbon dates ranging from 11.3 to 8.3kacal BP, with a cluster around 8.6kacal BP. This mass-wasting event has been unique since the emergence of its contributing catchment 0.2–0.3Ma ago. The low frequency of such an event could reflect the great resistance of rock mass in the source areas to weathering and erosion. The common blockage of valley floors by giant-boulder piles, which limits channel incision and sediment transport, could also increase the apparent stability of the mountain. The trigger of landslides in the Coastal Range has been linked to large earthquakes. Additionally, we propose that the great magnitude and duration of the observed early Holocene event were caused by the contemporaneous prolonged rainfall (and/or high frequency of typhoons) associated with the East Asian summer monsoon maximum. [Copyright &y& Elsevier]

We use field observations, laboratory experiments, and mathematical theory to characterize the full life cycle of a tributary-dammed lake. The natural lake formed in August 2007 at the confluence of the Laonong River, southern Taiwan, with a steep tributary. Due to heavy rains from typhoons Wutip and Sepat, the tributary delivered massive amounts of sediment to the confluence. This caused damming of the trunk river, the growth of a lake, and the demise of a bridge upstream of the confluence. The lake lasted until October 2007, gradually decaying due to incision and infill by subsequent stream flow. We reconstruct the episode using hydrological records, ground observations, and surveyed profiles. We then use the event to verify a mathematical theory of river and lake bed evolution. The theory is based on a diffusion description of river morphodynamics, constrained by backwater effects due to lake formation. After validation using laboratory experiments, the theory is applied to the Laonong field event. Good agreement is obtained between calculated and surveyed long profiles. The theory can also reproduce the complex stage hydrograph recorded upstream of the confluence. This suggests a role for simplified, but laboratory-tested mathematical theory in quantitative investigations of fluvial processes in the field. [ABSTRACT FROM AUTHOR]

The evidence of coseismic uplift on the dynamic, wave-dominated Hua-tung coast fringing the active Coastal Range (eastern Taiwan) has been equivocal, due to complex controls by wave and terrestrial sediment over morphological and ecological systems of the coast. This study, by applying radiocarbon dating methods, demonstrates coseismic-uplift nature of the coast by finding synchronously killed intertidal organisms (mostly boring shell Jouannetia sp.) stranded at different sites of the coast with distinct physiographic characters. Based on these data, together with evidence from wave-cut notch sequences, two coseismic-uplift systems are recognized. One centers around the northern-middle part of the coast and yields events with uplift amounts of maximal 3–6 m and an average recurrence interval of at least several hundred years. The most recent activity of this system, influencing at least 70 km of coast, occurred at ~0.9 ka. The earthquake generating this event also triggered extensive landslides/debris flows in the region. Another system, exemplified by the uplift associated with the 2003 Cheng-kung earthquake, centers on the southern part of the coast and yields uplift of likely <1 m every <0.2 ky. Two pre-historic events of this system are identified as occurring at ~0.7 ka and ~1.1 ka. These two coseismic-uplift systems are consistent in position with two anticlinal structures defined by long-term uplift of the coast. However, the areas subjected to maximal coseismic uplift are located off where the climaxes of long-term uplift occur, implying that the latter areas have been uplifted mainly by aseismic and/or relatively frequent/small-magnitude coseismic motion. [Copyright &y& Elsevier]

The article discusses the lateral erosion rivers being a significant control on the shape of channel cross sections and the union of channels and hillslopes. Recent observations connect lateral erosion to the variability of flow using two mechanisms were one is based on varying shear stress distributions within the channel with changing flood level while the other focuses on the competition between cover and tool effects that can be found in fluvial bedrock erosion. Details related to the rate of lateral erosion in bedrock channels and variability of river discharge can be attributed to the redistribution of boundary shear stress across a channel with changing flow stage.

We have discovered in a marine core, located 20 km east of the Coastal Range of Taiwan at the top of a 1200 m deep submarine high, sheltered from rivers discharges and gravitational flows, a 23 cm-thick anomalous sequence topped with broken bivalves and wood fragments. Based on radiocarbon dating, we distinguish five sub-events within ∼ 100 yrs, about 3000 years ago. The oldest four sub-events are interpreted as the record of local submarine landslides, likely triggered by clustered earthquakes that are common in this tectonically active area. The last event including shells and wood has a different origin. Despite the high level of climatic and tectonic recent activity, no comparable event has been observed at the same site since that time, making it unique during the last 3.75 ka. Hydrodynamical modeling stimulated with a set of submarine landslide-triggered tsunami sources and combined with the regional oceanic circulation dominated by the Kuroshio Current, allow us to better constrain the transport paths of the suspended material, the expected run-up at the coast, and ultimately the trigger and source that best fit our observations. The Kuroshio Current appears as a strong constraint that generally prevents cross-shore transport of suspended material even during a major tsunami. Since the largest aerial debris flow observed along the east coast of Taiwan during the last 4 ka was synchronous with the marine deposit, our preferred scenario starts with the occurrence of four distinct large earthquakes within less than a century, triggering both submarine and aerial landslides in the study area. Then, a giant typhoon reworked the nearshore material as well as the slided material stored upstream resulting in a debris flow. It deeply modified the surface current, allowing the wood debris and shells to fastly reach the core site. [ABSTRACT FROM AUTHOR]

We used multi-decadal pollen and diatom records from sediment core TFL-1 from Tsuifong Lake to reconstruct the vegetation dynamics and hydroclimate in northeastern Taiwan during the past 3500 cal BP. Coarse grained sediments in association with higher percentages of wetland pollen (Cyperaceae) and upper conifer pollen ( Tsuga and Pinus ) in the lower part of the core indicate low lake levels and a relatively cold/dry climate between 3500 and 2030 cal BP, reflecting a decline of the East Asian summer monsoon (EASM). Muddy sediments coupled with reduction of wetland pollen represent the rise of lake levels, implying the re-strengthening of the EASM during the past 2000 years. Paleotemperature was inferred from the variation of pollen origin from the upper and lower mountain forest, indicating the global temperature anomalies of the Medieval Warm Period (MWP) and the Little Ice Age (LIA). In comparison to the main climate forces in the North Pacific, we suggest that the long-term climatic trend in Taiwan was controlled by variations in EASM intensity, while increased precipitation over the past 2000 years may also be linked to warmer sea surface temperature (SST) of the western Pacific Warm Pool (WPWP) and increased El Niño-Southern Oscillation (ENSO) events, which increased typhoon intensity. Higher diatom-inferred pH during 2930–2030 cal BP and the LIA suggest strong hydrological disturbances, reflecting more typhoons passing over Taiwan. The frequent typhoon events could be linked by an abrupt shift of typhoon track, due to the reduction of the WPWP and expansion of the Northwestern Pacific High, which move typhoons in a more westerly direction. [ABSTRACT FROM AUTHOR]

Abstract: To understand whether soils on the same geomorphic surface have similar properties, this study examines the morphology of soils developed on the 30-kyr-old Taoyuan Terrace, Northwestern Taiwan. Relationships among soil morphology, variations in the groundwater table, and soil forming processes were established based on field investigations and previously published datasets. The soil series in the study area can be grouped into seven types, including three homogeneous groups, two mottled groups, one gley group, and one reworked group. Fifteen hand-core samples collected in the field and borehole data were used to understand spatial variations in soil morphology. The results indicate that soil morphology on the southern Taoyuan Terrace changes systematically from the fore-edge to the back-edge: from red/orange soils to yellow soils, then to mottled soils, and finally to gley soils. The borehole data indicate that the groundwater table is generally parallel to the ground surface but drops significantly near the terrace fore-edge. This pattern matches with the observed soil toposequence, indicating that the pedogenic processes in the study area are mainly controlled by groundwater distribution determined by topography. Although soils on the terrace fore-edge have developed under the well-drained conditions that favor lateritization, at least two distinct soil types (i.e., red and orange) can be observed there, reflecting different geomorphological settings in a paleo-fluvial landscape. This work suggests that geomorphological and hydrological conditions should be examined carefully before using soil morphology for correlating geomorphic surfaces. [Copyright &y& Elsevier]