Water and Sediment Discharge from Small Mountainous Rivers, Taiwan: The Roles of Lithology, Episodic Events, and Human Activities

Taiwan’s natural setting creates highly vulnerable watersheds whose rivers discharge disproportionately large quantities of sediment to the coastal ocean. The 16 Taiwanese rivers analyzed in this article discharge ∼180 Mt yr-1 of sediment to the coastal ocean, although totals over the past 20 years have varied between 16 and 440 Mt yr-1. The mean annual sediment yield of 9500 t km-2 yr-1 for the 16 rivers is 60-fold greater than the global yield of 150 t km-2 yr-1, but mean yields for the individual rivers vary by more than 2 orders of magnitude, from 500 to 71,000 t km-2 yr-1. Most sediment erosion and delivery occur in response to typhoon-generated floods, as evidenced by the fact that \u3e75% of the long-term flux occurs in \u3c 1% of the time, about one-third of which reaches hyperpycnal concentrations. Detailed analysis of the 16 watersheds reveals little evidence of any single environmental factor that controls sediment load. The Erren, the highest-yield river on Taiwan, drains an erodible but low-gradient watershed with relatively low runoff. In contrast, three east coast rivers, the Hoping, the Hualien, and the Beinan, have high sediment yields that may be explained by relatively frequent earthquakes coupled with high runoff. Farming and urbanization also have elevated sediment yields in eastern watersheds, whereas Holocene sediments buried in the Taiwan Strait suggest that present-day sediment loads of the western rivers may be no higher than prehuman levels

Calculating highly fluctuated suspended sediment fluxes from mountainous rivers in Taiwan

Small drainage basins, highly fractured rock, high relief, and steep gradients make Taiwan watersheds particularly sensitive to episodic events such as typhoons and earthquakes, and to various types of anthropogenic disturbance. Here we analyze the characteristics of a long-term hydrological dataset from Taiwan and re-evaluate methods used to calculate sediment loads for Taiwan\u27s event-driven rivers. We suggest using the rating curve method stratified down to seasonal levels to reflect the rapid changes in the relationship between water discharge and suspended sediment load. A program is developed to determine the optimal time-interval for constructing rating curves, and is used to calculate hourly, daily, yearly, and long-term mean suspended sediment loads. Seasonal rating curves applied to hourly discharges are particularly critical to calculate sediment fluxes and concentrations in response to episodic events, particularly typhoons. The calculated cumulative long-term mean sediment fluxes for the Jhou-Shuei and Bei-Nan Rivers are considerably smaller than those calculated using monthly weighted average (MWA) method (Dadson et al. 2003). The MWA method likely over-estimates the mean load due to more frequent sediment observations during high-flow events

Decline of Yangtze River water and sediment discharge: Impact from natural and anthropogenic changes

The increasing impact of both climatic change and human activities on global river systems necessitates an increasing need to identify and quantify the various drivers and their impacts on fluvial water and sediment discharge. Here we show that mean Yangtze River water discharge of the first decade after the closing of the Three Gorges Dam (TGD) (2003-2012) was 67 km(3)/yr (7%) lower than that of the previous 50 years (1950-2002), and 126 km(3)/yr less compared to the relatively wet period of pre-TGD decade (1993-2002). Most (60-70%) of the decline can be attributed to decreased precipitation, the remainder resulting from construction of reservoirs, improved water-soil conservation and increased water consumption. Mean sediment flux decreased by 71% between 1950-1968 and the post-TGD decade, about half of which occurred prior to the pre-TGD decade. Approximately 30% of the total decline and 65% of the decline since 2003 can be attributed to the TGD, 5% and 14% of these declines to precipitation change, and the remaining to other dams and soil conservation within the drainage basin. These findings highlight the degree to which changes in riverine water and sediment discharge can be related with multiple environmental and anthropogenic factors

Molecular and morphometric variation in European populations of the articulate brachiopod <i>Terebeatulina retusa</i>

Molecular and morphometric variation within and between population samples of the articulate brachiopod &lt;i&gt;Terebratulina&lt;/i&gt; spp., collected in 1985-1987 from a Norwegian fjord, sea lochs and costal sites in western Scotland, the southern English Channel (Brittany) and the western Mediterranean, were measured by the analysis of variation in the lengths of mitochondrial DNA (mtDNA) fragments produced by digestion with nine restriction endonucleases and by multivariate statistical analysis of six selected morphometric parameters. Nucleotide difference within each population sample was high. Nucleotide difference between population samples from the Scottish sites, both those that are tidally contiguous and those that appear to be geographically isolated, were not significantly different from zero. Nucleotide differences between the populations samples from Norway, Brittany, Scotland and the western Mediterranean were also very low. Morphometric analysis confirmed the absence of substantial differentiation

Intense hurricane activity over the past 1500 years at South Andros Island, the Bahamas

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in E. J., Donnelly, J. P., van Hengstum, P. J., Wiman, C., Sullivan, R. M., Winkler, T. S., d'Entremont, N. E., Toomey, M., & Albury, N. Intense hurricane activity over the past 1500 years at South Andros Island, the Bahamas. Paleoceanography and Paleoclimatology, 34(11), (2019): 1761-1783, doi:10.1029/2019PA003665.Hurricanes cause substantial loss of life and resources in coastal areas. Unfortunately, historical hurricane records are too short and incomplete to capture hurricane‐climate interactions on multi‐decadal and longer timescales. Coarse‐grained, hurricane‐induced deposits preserved in blue holes in the Caribbean can provide records of past hurricane activity extending back thousands of years. Here we present a high resolution record of intense hurricane events over the past 1500 years from a blue hole on South Andros Island on the Great Bahama Bank. This record is corroborated by shorter reconstructions from cores collected at two nearby blue holes. The record contains coarse‐grained event deposits attributable to known historical hurricane strikes within age uncertainties. Over the past 1500 years, South Andros shows evidence of four active periods of hurricane activity. None of these active intervals occurred in the past 163 years. We suggest that Intertropical Convergence Zone position modulates hurricane activity on the island based on a correlation with Cariaco Basin titanium concentrations. An anomalous gap in activity on South Andros Island in the early 13th century corresponds to a period of increased volcanism. The patterns of hurricane activity reconstructed from South Andros Island closely match those from the northeastern Gulf of Mexico but are anti‐phased with records from New England. We suggest that either changes in local environmental conditions (e.g., SSTs) or a northeastward shift in storm tracks can account for the increased activity in the western North Atlantic when the Gulf of Mexico and southeastern Caribbean are less active.This work was funded by the National Science Foundation Graduate Research Fellowship (to E.J.W.), National Science Foundation grant OCE‐1356708 (to J.P.D. and P.J.vH.), the Dalio Explore Foundation and the USGS Land Change Science Program (M.R.T.). We are grateful to members of WHOI Coastal Systems Group, in particular Stephanie Madsen, for their help in the processing core samples. We thank two anonymous reviewers, Matthew Lachniet, Marci Robinson (USGS) and Miriam Jones (USGS) for their helpful feedback on earlier versions of this manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. The data are available on the National Climatic Data Center (http://www.ncdc.noaa.gov/dataaccess/paleoclimatology‐data) and WHOI Coastal Systems Group (https://web.whoi.edu/coastal‐group/) websites