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From 1996 through 2007, several heavy typhoons and earthquakes precipitated landslide hazards in southeast Taiwan. In the present study, we analyze the impact of topography, lithology, rainfall and earthquakes on landsliding and sediment transport by quantifying the landslide ratio (ratio of landslide area to catchment area) and the sediment discharge for the Sinwulu and Luye upstream catchments of the Peinan River. The steep topography in these two catchments causes a large proportion of landslide ratios on slopes to exceeding 50°, a condition which accelerates the delivery of landslide debris to the downslope channel. The landslide ratios of 0.84%–2.30% are obviously related to the rock strength and discontinuity density of the terrain, but we find the magnitude of landsliding and sediment discharge to vary with the occurrence of earthquakes. Typhoon-induced rainstorms generate the bulk of sediment discharge in the study catchments, though the landslide debris is transported by the huge amounts of runoff produced by rainstorms. A contributing factor is the recent occurrence of severe earthquakes which weakened the geomaterial, leading in turn to further increases of fluvial sediment during subsequent typhoons. Additionally, the finding that eventual reduction of unit sediment concentration after high magnitude earthquake events reveals that the downslope transport of landslide debris associated with earthquake activity and subsequent typhoon events precipitates a relatively short term, 1–3 year, increase of erosion rates in these two study areas in southeastern Taiwan.

Typhoon-triggered landslides deliver huge amounts of sediment to the upstream channel of the Shihmen Reservoir in northern Taiwan. Observation data regarding landsliding, sediment discharge and water turbidity following five major typhoon events from 1985 to 2006 demonstrated that each time water turbidity in the reservoir area rapidly increased up to ten-fold from the river catchment drainage, and the weight of landslide debris exceeded total sediment discharge five-fold. The fact that huge amounts of landslide debris still remained on upstream slopes and water turbidity suddenly increased in the reservoir area but not in upstream channel implied that the increasing water turbidity in the Shihmen Reservoir was indirectly related to the large landslides occurring in the upstream catchment. The main cause of high turbidity in the reservoir area was that, during a typhoon event, high water discharge flowing into the reservoir scoured the fine fraction sediment at the bottom of the reservoir and formed hyperpycanl flow with high turbidity, which then ascended to contaminate the reservoir surface water.

The transfer of organic carbon from the terrestrial biosphere to the oceans via erosion and riverine transport constitutes an important component of the global carbon cycle. Measurements of particulate organic carbon load and composition in the LiWu river, Taiwan, during cyclone-triggered floods suggest that tropical cyclones may facilitate the delivery of non-fossil particulate organic carbon to the ocean and its subsequent burial. The transfer of organic carbon from the terrestrial biosphere to the oceans via erosion and riverine transport constitutes an important component of the global carbon cycle1,2,3,4. More than one third of this organic carbon flux comes from sediment-laden rivers that drain the mountains in the western Pacific region3,5. This region is prone to tropical cyclones, but their role in sourcing and transferring vegetation and soil is not well constrained. Here we measure particulate organic carbon load and composition in the LiWu River, Taiwan, during cyclone-triggered floods. We correct for fossil particulate organic carbon using radiocarbon, and find that the concentration of particulate organic carbon from vegetation and soils is positively correlated with water discharge. Floods have been shown to carry large amounts of clastic sediment6. Non-fossil particulate organic carbon transported at the same time may be buried offshore under high rates of sediment accumulation7,8,9. We estimate that on decadal timescales, 77–92% of non-fossil particulate organic carbon eroded from the LiWu catchment is transported during large, cyclone-induced floods. We suggest that tropical cyclones, which affect many forested mountains within the Intertropical Convergence Zone10, may provide optimum conditions for the delivery and burial of non-fossil particulate organic carbon in the ocean. This carbon transfer is moderated by the frequency, intensity and duration of tropical cyclones.

A growing literature deals with bedrock channels, erosion processes within them, and their role in landscape evolution. Formal definitions currently in use classify channels as bedrock when alluvial cover is discontinuous or thin. This is equated with a physical property of the flow. Using up-to-date erosion laws and descriptions of channel dynamics, we show that existing definitions are difficult to apply and may lead to conflicting classifications. We propose the definition: A bedrock channel cannot substantially widen, lower or shift its bed without eroding bedrock. Applied to channels in Taiwan, this leads to a different classification than previous definitions. By analysing at-a-station hydraulic geometry of these channels, we show that the classification exposes different controls of sediment effects on dynamics of channels classified as alluvial and bedrock.

The aim of this paper is to present data on the occurrence of landslides triggered by typhoon and earthquake events, and to discuss the basic earthquake triggering mechanisms involved. Four events of typhoon and earthquake triggered landslides in the Chenyoulan River catchment of central Taiwan during 1996 to 2004 were studied in order to identify their controlling factors. The landslides areas were measured by comparing aerial photos taken over the past 18 years, and sediment discharge was counted at hydrometric stations. Investigation results demonstrate that hillslope geomaterials that had been disturbed by strong seismic shaking induced expansion of landslide areas during subsequent events. Analysis of Normalized Difference Vegetation Index (NDVI) also registered the changes of uncovered conditions on the hillslope caused by landsliding. When flow discharge is lower than 100 m 3 s − 1 , post-seismic sediment concentration is roughly four times higher than pre-seismic concentration with the same flow discharge. This fate indicates that the amount of sediment discharge at normal flow discharge is affected by supplied sediment volume, which tends to increase with seismic activity.

Groundwater chemical fluxes from the Pingtung Plain in SW Taiwan to the ocean were determined by analysing waters from 43 wells at varying depths through a 237 m deep window across the Pingtung Plain, for major dissolved cations, anions, dissolved SiO2, and stable isotopic composition of O and H, and computing their subsurface water fluxes from measurements of hydraulic heads and formation permeabilities. The results show that between 1.5% ( SO 4 2 - ) and 12.3% (Ba2+) of the total chemical weathering flux discharged to the ocean (Kaoping River combined with groundwater) can be attributed to the groundwater. Estimated propagated errors at 1σ on subsurface fluxes are ±20%. Multi-year daily hydraulic head data give the direction of groundwater flow through the plain, and indicate that pumping has led to episodic reversals of flow, facilitating seawater intrusion in the near-coast aquifer. Tracing end-member proportions using mixing relationships shows that, in addition to seawater and meteoric water, hot-spring activity contributes to the dissolved chemistry of the groundwater. In addition to these three end-members, the weathering of carbonate and silicate minerals in the plain accounts for the remainder of the chemical budget. Hydrological connectivity exists throughout the drilled depth of the basin, but chemical gradients show that flow is stratified, with up to a twofold increase in silicate-derived Na+ seen in deeper horizons as compared to the near surface. For all ions except SO 4 2 - , the average concentrations of dissolved species in the coastal groundwaters exceed those of the river, ranging from a factor of 1.33 in the case of Li+ to 27.29 in the case of Cl−. The results suggest that submarine groundwater chemical fluxes through the drilled depth of the Pingtung Plain into the Taiwan Strait are modest in comparison to those related to surface runoff.

[1] Erosion of particulate organic carbon (POC) occurs at very high rates in mountain river catchments, yet the proportion derived recently from atmospheric CO2 in the terrestrial biosphere (POCnon-fossil) remains poorly constrained. Here we examine the transport of POCnon-fossil in mountain rivers of Taiwan and its climatic and geomorphic controls. In 11 catchments we have combined previous geochemical quantification of POC source (accounting for fossil POC from bedrock), with measurements of water discharge (Qw) and suspended sediment concentration over 2 years. In these catchments, POCnon-fossil concentration (mg L−1) was positively correlated with Qw, with enhanced loads at high flow attributed to rainfall driven supply of POCnon-fossil from forested hillslopes. This climatic control on POCnon-fossil transport was moderated by catchment geomorphology: the gradient of a linear relation of POCnon-fossil concentration and Qw increased as the proportion of steep hillslopes (>35°) in the catchment increased. The data suggest enhanced supply of POCnon-fossil by erosion processes which act most efficiently on the steepest sections of forest. Across Taiwan, POCnon-fossil yield was correlated with suspended sediment yield, with a mean of 21 ± 10 tC km−2 yr−1. At this rate, export of POCnon-fossil imparts an upper bound on the time available for biospheric growth, of ∼800 yr. Over longer time periods, POCnon-fossil transferred with large amounts of clastic sediment can contribute to sequestration of atmospheric CO2 if buried in marine sediments. Our results show that this carbon transfer should be enhanced in a wetter and stormier climate, and the rates moderated on geological timescales by the regional tectonic setting.

Large earthquakes deform the Earth's surface and drive topographic growth in the frontal zones of mountain belts. They also induce widespread mass wasting, reducing relief. The sum of these two opposing effects is unknown. Using a time series of landslide maps and suspended sediment transport data, we show that the MW7.6 Chi-Chi earthquake in Taiwan was followed by a period of enhanced mass wasting and fluvial sediment evacuation, peaking at more than five times the background rate and returning progressively to pre-earthquake levels in about six years. Therefore it is now possible to calculate the mass balance and topographic effect of the earthquake. The Choshui River has removed sediment representing more than 30% of the added rock mass from the epicentral area. This has resulted in a reduction of surface uplift by up to 0.25 m, or 35% of local elevation change, and a reduction of the area where the Chi-Chi earthquake has built topography. For other large earthquakes, erosion may evolve in similar, predictable ways, reducing the efficiency of mountain building in fold-and-thrust belts and the topographic expression of seismogenic faults, prolonging the risk of triggered processes, and impeding economic regeneration of epicentral areas.

Mountain building exposes fossil organic carbon (OC fossil ) in exhumed sedimentary rocks. Oxidation of this material releases carbon dioxide from long-term geological storage to the atmosphere. OC fossil is mobilized on hillslopes by mass wasting and transferred to the particu- late load of rivers. In large fl uvial systems, it is thought to be oxidised in transit, but in short, steep rivers that drain mountain islands, OC fossil may escape oxidation and re-enter geological storage due to rapid fl uvial transfer to the ocean. In these settings, the rates of OC fossil transfer and their controls remain poorly constrained. Here we quantify the erosion of OC fossil from the Taiwan mountain belt, combining discharge statistics with measurements of particulate organic carbon load and source in 11 rivers. Annual OC fossil yields in Taiwan vary from 12 ± 1 to 246 ± 22 tC km −2 yr −1 , controlled by the high physical erosion rates that accompany rapid crustal shortening and frequent typhoon impacts. Effi cient transfer of this material ensures that 1.3 ± 0.1 ◊ 10 6 tC yr −1 of OC fossil exhumed in Taiwan is delivered to the ocean, with

Small rivers draining mountain islands are important in the transfer of terrestrial particulate organic carbon (POC) to the oceans. This input has implications for the geochemical stratigraphic record. We have investigated the stable isotopic composition of POC (δ13Corg) in rivers draining the mountains of Taiwan. In 15 rivers, the suspended load has a mean δ13Corg that ranges from −28.1±0.8‰ to −22.0±0.2‰ (on average 37 samples per river) over the interval of our study. To investigate this variability we have supplemented suspended load data with measurements of POC in bedrock and river bed materials, and constraints on the composition of the terrestrial biomass. Fossil POC in bedrock has a range in δ13Corg from −25.4±1.5‰ to −19.7±2.3‰ between the major geological formations. Using coupled δ13Corg and N/C we have found evidence in the suspended load for mixing of fossil POC with non-fossil POC from the biosphere. In two rivers outside the Taiwan Central Range anthropogenic land use appears to influence δ13Corg, resulting in more variable and lower values than elsewhere. In all other catchments, we have found that 5‰ variability in δ13Corg is not controlled by the variable composition of the biomass, but instead by heterogeneous fossil POC. In order to quantify the fraction of suspended load POC derived from non-fossil sources (Fnf) as well as the isotopic composition of fossil POC (δ13Cfossil) carried by rivers, we adapt an end-member mixing model. River suspended sediments and bed sediments indicate that mixing of fossil POC results in a negative trend between N/C and δ13Corg that is distinct from the addition of non-fossil POC, collapsing multiple fossil POC end-members onto a single mixing trend. As an independent test of the model, Fnf reproduces the fraction modern (Fmod) in our samples, determined from 14C measurements, to within 0.09 at the 95% confidence level. Over the sampling period, the mean Fnf of suspended load POC was low (0.29 ± 0.02, n = 459), in agreement with observations from other mountain rivers where physical erosion rates are high and fossil POC enters river channels. The mean δ13Cfossil in suspended POC varied between −25.2±0.5‰ and −20.2±0.6‰ from catchment to catchment. This variability is primarily controlled by the distribution of the major geological formations. It also covers entirely the range of δ13Corg found in marine sediments which is commonly thought to derive from mixing between marine and terrigenous POC. If land-sourced POC is preserved in marine sediments, then changes in the bulk δ13Corg observed offshore Taiwan could instead be explained by changes in the onshore provenance of sediment. The range in δ13Corg of fossil organic matter in sedimentary rocks exposed at the surface is large and given the importance of these rocks as a source of clastic sediment to the oceans, care should be taken in accounting for fossil POC in marine deposits supplied by active mountain belts.

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.

[1] The magnitude-frequency distribution of boundary shear stress frames erosion rates in bedrock rivers, but empirical constraints are rare, particularly for extreme floods. Here we present measurements of mean stress τb and its frequency distribution along a fast-eroding river in Taiwan. We construct rating functions of discharge and hydraulic geometry using high-resolution satellite images of flood patterns, river stage time-series, topographic profiles, and post-flood field surveys. The method allows us to assess the spatiotemporal variation in τb along the channel. The boundary shear stress PDF p(τb) has a steep power-law tail, and includes semiannual floods generating τb ≈ 100–200 Pa and 50-year, 3000 m3 s−1 events driving τb ≥ 300 Pa. All such floods contribute to modification of the coarse alluvial cover and erosion of the bedrock bed. Given the steep tail decay in p(τb), the rapid channel incision probably owes more to the exceptional frequency of moderate shear stresses than to the magnitude of the extremes.

[1] The impact of tropical cyclones on precipitation variability and surface processes in Taiwan is studied using suspended sediment and river discharge data along with a meteorological model of tropical cyclone dynamics. The interactions between tropical cyclones and topography produce distinctive, localized patterns of heavy precipitation that can trigger high sediment discharge in Taiwan’s rivers. We focus on Typhoon Toraji, which made landfall in Taiwan on 30 July 2001 and was the first major tropical cyclone to affect Taiwan after the 1999 Chichi earthquake. The passage of the tropical cyclone eye over the east coast of Taiwan produced intense rainfall (>100 mm h 1 ) for a short period (about 2 hours), triggering the highest sediment concentration ever recorded on the Hualien River. Orographic effects localized heavy rainfall (between 10 and 50 mm h 1 ) over the southwestern slopes of the Central Mountain Range, triggering high sediment discharge on the Kaoping River and flushing landslide debris produced during the Chichi earthquake into the Choshui River. We show that a range of atmospheric processes, with distinctive spatial variability and varying degrees of coupling to topography, interacted with tectonic processes of limited spatial extent to produce the overall sediment discharge from Taiwan during Typhoon Toraji. Landscape evolution models may need to be modified to account for the distinctive patterns of localized rainfall in tropical mountain belts. Furthermore, the links between tropical cyclones and topography suggest the potential for a range of previously unrecognized feedbacks between tectonics and climate in tropical mountain belts.

Rivers draining the tectonically active island of Taiwan commonly discharge suspended sediment to the ocean at hyperpycnal concentrations (>40 kg m−3), typically during typhoon-driven floods. During the period 1970–1999, between 99 and 115 Mt yr−1 of sediment was discharged at hyperpycnal sediment concentrations from Taiwan to the sea. This amount represents 30–42% of the total sediment discharge from Taiwan to the ocean. The spatial distribution of hyperpycnal discharge broadly mirrors the pattern of total sediment discharge, and rivers draining catchments having recent earthquakes and weak rocks, such as the Choshui and Erhjen, discharge up to 50–70% of their sediment at hyperpycnal concentrations. Following the Chi-Chi earthquake, the frequency of hyperpycnal flows increased, because of an earthquake-driven increase in sediment supply. Landslides triggered by the Chi-Chi earthquake have resulted in an increase in the concentration of suspended sediment in rivers for a given water discharge. In turn, the threshold flood discharge required to generate hyperpycnal flow has decreased, and so hyperpycnal flows are occurring more frequently. Our findings suggest that if hyperpycnal plumes evolve into bottom-hugging gravity currents descending to and ultimately debouching in the deep sea, earthquakes may be recorded as bundles of turbidites.

In tectonically active mountain belts, earthquake-triggered landslides deliver large amounts of sediment to rivers. We quantify the geomorphic impact of the 1999 Mw 7.6 Chi-Chi earthquake in Taiwan, which triggered >20,000 landslides. Coseismic weakening of substrate material caused increased landsliding during subsequent typhoons. Most coseismic landslides remained confined to hillslopes. Downslope transport of sediment into the channel network occurred during later storms. The sequential processes have led to a factor-of-four increase in unit sediment concentration in rivers draining the epicentral area and increased the magnitude and frequency of hyperpycnal sediment delivery to the ocean. Four years after the earthquake, rates of hillslope mass wasting remain elevated in the epicentral area.

The erosion of mountain belts controls their topographic and structural evolution1,2,3 and is the main source of sediment delivered to the oceans4. Mountain erosion rates have been estimated from current relief and precipitation, but a more complete evaluation of the controls on erosion rates requires detailed measurements across a range of timescales. Here we report erosion rates in the Taiwan mountains estimated from modern river sediment loads, Holocene river incision and thermochronometry on a million-year scale. Estimated erosion rates within the actively deforming mountains are high (3–6 mm yr-1) on all timescales, but the pattern of erosion has changed over time in response to the migration of localized tectonic deformation. Modern, decadal-scale erosion rates correlate with historical seismicity and storm-driven runoff variability. The highest erosion rates are found where rapid deformation, high storm frequency and weak substrates coincide, despite low topographic relief.