Middle–late Holocene river terraces in the Erhjen River Basin, southwestern Taiwan—implications of river response to climate change and active tectonic uplift

Abstract

We reconstruct the Holocene river history of the Erhjen River (area: 140 km 2) by correlating river terraces aided by 28 radiocarbon dates. Multiple terraces developed in the lower Erhjen River since the middle Holocene; they converge downstream to the Coastal Plain. The rates of channel incision into bedrock calculated from these terraces at Yuehshihchieh are 7–8 mm/year during ca. 5.7–2.5 ka, 5 cm/year during 1.5–1.3 ka, and 1 cm/year since 1 ka; the last is close to the average incision rate since middle Holocene. Meanwhile, only a single but wide middle–late Holocene paleo-floodplain was developed in the upper Erhjen River; it was completely abandoned only after 0.8 ka, likely following an episode of base-level fall starting from the Coastal Plain. Based on the apparent downstream and upstream convergence of these dated terraces, we identify a doming structure or anticline within the basin, which results in a tilt rate of 10 −6 to 10 −7 per year in the lower Erhjen River valley. The major terraces here had different initial long profiles, which implies that a critical (graded) long profile may not be a prerequisite for formation of a wide erosional terrace surface. Instead, we propose that these terraces were initiated by a series of catastrophic rainfall events, probably climatic-related, which brought a large amount of bedload from hillslopes to prevent the channel from incising when valley widening was facilitated by high-discharge runoff. We find that not only terrace-surface formation but also channel incision can be strongly controlled by climatic-driven discharge and bedload conditions, as suggested by the contrast of bedrock incision rates we observe at Yuehshihchieh. Apparently, the climate and its effects on the landscape cannot be regarded as constant during the Holocene even in a humid tropical area like Taiwan. However, such a fluctuation of climate could only be recorded in a setting where rivers have a high tendency to incise so that multiple terraces can be created. The lower Erhjen River that is characterized by active tectonic tilting is an example of this setting.