Prolonged Response of River Terrace Flooding to Climate Change

Abstract

From the start of river incision onward, the abandoned terrace surface is only reached by floods during peak discharges. Two main flood facies are distinguished: a relatively high-energetic, coarse-grained facies and a relatively low-energetic, fine-grained facies. In general, the flood deposits become gradually finer-grained and the finer-grained facies relatively more prominent when the river incises progressively deeper. This signifies a delayed and prolonged effect of channel incision and flood deposition compared with the climate changes that initiated the incision. However, these long-term trends may be interrupted by shorter-term events of flooding or non-deposition. Those short events are expressed by cycles of coarse-grained deposits from small/shallow flooding channels due to short peak discharges or fine-grained suspended sediment and incipient soils during periods of low flow. These short events may be attributed to short climatic episodes or intermittent intrinsic river evolution.

1. Introduction

Traditionally, terrace deposits are presented with a lower main fluvial unit consisting of coarse-grained, often gravelly or sandy sediments abruptly separated from an overlying unit consisting of fine-grained, mostly silty sediments [1,2,3,4]. The latter sediments are often interpreted as the result of fluvial reworking, but occasionally also as windblown loess. This sedimentary transition may simply be related to the termination of channel deposition (gravel unit) and the onset of river incision that marks the initiation of terrace formation and subsequent floodplain development with deposition of fine-grained sediments [5,6]. However, a series of specific questions remain open.

A first question arises of whether there is one simple break in energy conditions and river dynamics, or if the transition is gradual. Therefore, the floodplain composition and internal structural diversity are studied (e.g., [7,8]). Next, a detailed examination of the significance of the changing lithology, ultimately with silty deposits replacing gravel deposition towards the end of terrace formation, is needed. Secondly, external factors, such as climate possibly interrelated with tectonic and/or intrinsic control, are usually held responsible for triggering river incision (e.g., [5,9,10,11,12,13,14,15,16,17]). But it remains a pertinent question how a relatively abrupt (climate) trigger is expressed in the responding fluvial morphology and sedimentology [5,6,15,18,19]. More precisely, does the river system react simultaneously with the external trigger, or may any delay and non-linear effect be involved in the evolution of the sedimentary environment of the (abandoned) floodplain (e.g., [20,21,22])? It is not the intention here to describe the diversity in floodplain deposits which has been done already, for instance [1,23,24]. Instead, the temporal evolution and specifically the prolonged and delayed reaction of the river system at the waning and final phase of terrace formation compared with the triggering climatic event are illustrated and discussed in the following cases.

2. Study Sites

The described objectives may well be studied in large, regionally significant river systems with periodically high discharges and sediment loads. Although similar processes also occur in smaller systems, they are often less expressed and have a weaker resolution due to local factors (such as Dinkel and Mark, in the Dutch small river systems. Terrace gravel is common in many fluvial systems. Examples of a full succession of terminal terrace gravel deposition can be found in China, in subtropical environments. Here, four reported sedimentary sites from the Yellow River and Yangtze River are summarized, including the well-documented sediment sequences in the Huangshui system (a main tributary of the Yellow River) at Ledu [25] and at Ping’an (Figure 1) [26]. Although the upper coarse-grained deposits at the latter site represent an alluvial fan rather than a river channel, the overlying fine-grained deposits are similar. The third case represents a rich variety of floodplain deposits, superposed on channel gravels, which has been recently illustrated in detail from the middle Pleistocene terraces of the Hanjiang River, a major tributary of the Yangtze River, for instance, at Jinxing (Figure 1) [27,28] and neighbouring sites [29]. The final site at Silong is also from a Yellow River terrace, east of Lanzhou, described in detail by [30]. A series of OSL dates from all the sites enables us to chronologically frame the floodplain evolution within a paleoclimatic context.

3. Basic Sedimentary Data

3.1. Ledu and Ping’an Key Sites (Huangshui River, Yellow River Basin)

Well-rounded gravel (Gt, Gp) with interbedded coarse sand (St) is typical of braided river terrace deposits [25]. An abrupt separation between such gravel deposits and overlying fine-grained deposits is quite common and is generally assumed to coincide with a sedimentary hiatus. In conditions of continued sedimentation, the sedimentary transition is typically more gradual. It is usually expressed by smaller-sized channels with pebbles of smaller diameter at the bottom and the interfingering of gravel layers with silty beds towards the top. A typical example of the final coarse-grained terrace deposits can be found at the Ledu site of the Huangshui River (Figure 2). This is the result of decreasing energetic conditions due to the approaching abandonment of the terrace by channel entrenchment or lateral migration.

Sediment aggraded from a high- to a low-energy alluvial fan and finally to a floodplain at the Ping’an site in the Xining basin [26]. It is probably connected to a terrace 26 m above the present floodplain (apf). Two facies associations were distinguished in the fan on top of the terrace gravel: (facies 1) the lower part of the section shows a high-energy association with matrix- and clast-supported, poorly sorted, planar cross-stratified and crudely stratified sheets of coarse-grained sediments (Gh), interpreted as highly concentrated flow deposits (Figure 3); (facies 2) higher up in the section, a low-energy association of horizontal laminated sand (Sh) and interbedded laminated silt and sand occurs, with vertically stacked aeolian sand mounds and dispersed gravel lenses, interpreted as deposits in a braided-stream and sheet-flood environment (Figure 3; [26]). Highly competent flow in the upper flow regime (facies 1) was followed by lower energy accumulation in repeated flood pulses, with occasional gravel from flash flooding (facies 2). Towards the top of the fan, streamflow capacity decreased, resulting in the deposition of lenticular gravel beds in minor, shallow channels. The sandy fan aggradation was episodically interrupted by aeolian mound accumulations and weak pedogenesis (Figure 3).

The uppermost sediments (facies 3) are composed of massive silt (Fm) with incipient palaeosols, interpreted as the result of settling from floodwaters and weak pedogenesis (Figure 3). River incision may have commenced already during the fan deposition, whereas facies 3 may reflect occasional flooding that topped the fan lobe.

According to the OSL dates (Figure 3), the fan was rapidly deposited during the late MIS 6 (between ~168 and 126 ka) [26]. It was covered by flood loam and shortly interrupted by soil formation in a warming setting towards the end of that glacial period [32], that is, at the transition from MIS 6 to MIS 5 (between c. 145 and 125 ka).

3.2. Jinxing and Nearby Sites along the Hanjiang River (Yangtze River Basin)

Typical floodplain deposits of the Hanjiang River are primarily composed of two facies [28]. The sedimentary structures in the relatively high-energy floodplain environment (facies b), e.g., trough crossbedding and occasionally finely laminated sands and silts, point to deposition in small, laterally migrating channels, bars, and sand sheets (Figure 4). This facies generally interfingers with aggrading horizontal parallel sheets of overbank deposits with occasional ripples in alluvial pools and swamps in a floodplain environment with much lower energy (facies c). The latter facies points to a generally calmer depositional environment than facies b, occasionally terminating in emergent phases and soil formation (Figure 4 and Figure 5A). These floodplain deposits closely resemble the ‘medium-energy non-cohesive floodplains’ of facies b, and the ‘low-energy cohesive floodplains’ of facies c of [33], respectively.

Three groups of grain-size populations can be identified based on their modal values (Figure 5B): a fine clay fraction (1), a medium silt fraction (2), and a medium-to-coarse sand fraction (3). The finest fraction (1) with a diameter size below 5 μm is present in all samples, albeit in small amounts. It was likely deposited as strongly weathered sediment that was hydrodynamically sorted in standing water in abandoned pools on the floodplain. The coarsest fraction (3) is also common, with a modal size ranging from 225 to 800 μm, and often accompanied by pebbles. The silt fraction (2) is more complex and may be further subdivided into subfractions between 5 and 37 μm, representing a continuum in windblown loess. This loess is deposited primarily from near-surface or low-suspension clouds by monsoonal winds, and from high-suspension subfractions clouds by westerlies [34,35]. The finest subfractions (5–8 μm) are mostly only visible as a ‘shoulder’ or asymmetric bulge in the frequency distribution curves (Figure 5B). The original loess was transported by wind and subsequently reworked and redeposited in a floodplain environment without changing its general grain-size distribution [27,35]. The finest subfraction likely has a reworked and pedogenic origin.

Almost all sediments contain specific amounts of all three main fractions (Figure 5B). However, the proportions of those fractions are obviously specific to the two facies: facies b is generally dominated by the coarse fraction (3) pointing to transport as bedload in channels or as sheetwash, whereas reworked loess fraction (2) forms the main part of facies c. Flooding inundated the terrace generally in cyclic successions, from high-energy to low-energy floodplain sedimentation, terminating with stability and incipient soil formation. A general decline in energy conditions is not obvious at this site because the uppermost part of the section is apparently mixed up with coarse-grained slope or fan deposits from the valley sides, containing angular clasts of local origin. Nevertheless, the highest energy conditions occurred at the very start of the floodplain deposition where the sand content is >60%, with an average modal grain size of 400 μm or more.

Other Terraces in the Hanzhong Basin

Nearby sections with sediments that cover the terrace gravels along the Hanjiang River were described by [27,29] using grain size and shape of the sediment particles. According to these authors, the general sedimentary evolution shows gradually decreasing fluvial energy starting with cross-bedded sandy silt in shallow channels at the transition from a fluvial to a floodplain environment. They are overlain by silty floodplain sediment of variable thickness that is gradually fining upward from a high-energy floodplain situation to a low-energy floodplain environment with increasing aeolian input [27]. The uppermost deposit, locally up to 20 m thick [29], consists of aeolian loess. It is interbedded with palaeosol(s) and sediments that are the result of episodic surface runoff as evidenced by the presence of coarse sand grains. The grain-size characteristics of this ‘aeolian’ deposit highly resemble the ‘low-energy floodplain facies c’ of Jinxing.

3.3. Silong Site (Yellow River Basin)

At Silong, the gravel unit of the second terrace of the Yellow River is at ~8 m apf (Figure 6 [30]. Overlying floodplain deposition starts with a 0.5–0.8 m thick unit (unit B) consisting of 2–5 cm thick alternating horizontally bedded laminae or climbing ripples (Sh and Sr) of very fine sand to coarse-grained silt (Figure 6A) (similar to facies c at Jinxing) and coarse sand with current ripples and occasional cross-bedding (similar to facies b at Jinxing;). Facies c (samples SL-B1 and SL-B4 in Figure 6D) has a major mode of 55 μm and, occasionally, an additional coarser fraction (sample SL-B1). Facies b (samples SL-B2 and SL-B3 in Figure 6D) has a major fraction of mode size at 287 μm with a minor amount of fine sandy silt and a very small amount of fine silt. All grain-size distribution curves are similar to those of the Jinxing site, with a slight clay admixture. The top of this unit is brownish due to weak pedogenesis. Both facies represent poorly sorted floodplain deposits resulting from different energy conditions. Facies b is almost identical to channel fills that are present within the lowermost gravel body (unit A), whereas the fine-grained facies c shows the grain-size characteristics of a reworked loess transported in suspended flow (unit B) [35]. The overlying unit C is a massive, loose, homogenous, gray, very well sorted, very fine, silty sand (Sm) with a modal value of 65 μm and incipient soil formation at the top (Figure 6B,C). In addition to its dune morphology, the equal grain-size distribution from top to bottom in the ~3 m thick sediment unit indicates relatively equal transport energy confirming its aeolian dune origin. The top unit D shows sedimentary structures identical to unit B. This demonstrates that after an aeolian phase (unit C), the site was invaded again by the river, more specifically in an overbank position. The grain-size frequency distribution is slightly different from that of unit B by the absence of the coarse facies b (Figure 6B). This means that the flood sediments consist only of the suspended fraction (13–29 μm modal size) which is even smaller sized than in unit B. This proves a degradation in energy conditions of the flooding process due to the deeper position of the active channel. The fining-upward sedimentary succession continued with a several-meter-thick, typical aeolian loess at the top of the section on which the present soil is formed.

A series of OSL dates (Figure 6A) reflect the succession in the history of the termination of terrace formation. The lowermost sample (SL-1) is from a sand lens at the top of the gravel layer, dated to ~23.6 ka (Figure 6A). Other samples date the floodplain and dune sediments on top of the terrace, starting from unit B, dated at ~18.9 ka, to the overlying sand dune (unit C) with an age between ~20.7 and ~16.0 ka, and the laminar flood loam (unit D) was dated at ~14.8 and ~13.2 ka (Figure 6A). This means that the braided river that formed the gravel layer below the terrace surface was active during the last glacial maximum (LGM) and the terrace surface invasion by flooding started from the end of the LGM onwards. The flooding was terminated at the transition to the topmost loess layer, for which no OSL age is available but is most probably dated from the ultimate phase of last glacial loess-to-deglacial loess deposition and terminated by the Holocene soil.

4. Discussion

As is apparent from many sections, multiple sedimentary facies can be distinguished overlying the terrace surfaces. The general upward-fining succession at the different described sites at the end of terrace/fan deposition (Ping’an and Ledu) and subsequently from high- to low-energy flood facies associations is very characteristic in all sections. In general, two main facies of flood deposits are described with many intermediaries. They are characterized by various grain-size distributions with equivalent energy conditions. In all cases, the flooding deposition process was occasionally stopped, leading to the formation of primary soils or aeolian dunes between flood sequences. The general fining-up tendency in channel/fan and flood deposits reflects a slowing down of flow intensity. On a smaller scale, recurrent flooding often occurred during the floodplain accretion at the top of the terraces, resulting in cyclic patterns of (relatively short) sedimentation with individual fining-up patterns within each cycle.

The well-dated fining-up tendency observed in the post-LGM deposits at Silong and at the end of MIS-6 at Ping’an provides an opportunity to link changing depositional conditions to the supposed triggering mechanism, i.e., climate. It is striking that the sedimentary transition started when climatic conditions became warmer after a period of maximum cold. At Silong, it occurred after MIS-2 or LGM towards the Holocene, whereas a similar evolution took place at Ping’an at the decline of the penultimate glacial period (~MIS-6) towards the climate warming of the last interglacial (~MIS-5). It has frequently been found that evolution to climatic warming provided the required conditions for general river incision both in temperate and monsoonal regimes (e.g., [5,6,9,10]). In specific cases, this incising process could take place relatively quickly, for instance, in the order of a couple of thousand years, on certain easily erodible subsoil, without flooding the former floodplain. However, the rate of incision is generally dependent on local topographical or climatic conditions [36,37]. Specifically in the described Chinese monsoonal systems, the river incision process was relatively arduous, thus it was more gradual and needed more time than in easily erodible substrate. This provided the opportunity for the river to temporarily invade the former floodplain during the periods of peaked discharges. Progressive incision resulted in a gradually deeper position of the river concerning its initial floodplain, and thus the intensity of the flooding process should have decreased equally. Such fluvial evolution explains the general fining-up tendency of the flood sediments, but the former terrace surface was also invaded repeatably with intermittent short phases of non-deposition. This short cyclicity is superposed on the gradual fining-up tendency of longer duration.

The detailed chronological control of the flood deposits at the Silong and Ping’an sites enables us to link the described fluvial processes of incision and flooding to changed climatic conditions. In fact, climate warming took place relatively quickly compared to the flooding response which was much more gradual and extended over a much longer time [38,39,40]. Thus, the response of the sedimentary succession of river deposition during and after the abandonment of the terrace was slower than the triggering event. In that sense, it was prolonged and delayed with regard to the relatively abrupt climatic warming events after LGM (MIS-2) at Silong and the maximum cold during MIS-6 at Ping’an. Finally, this retarded flooding evolution means that the non-linear river reaction to climatic changes was not only responsible for the initiation of river incision at climatic transition, as previously demonstrated (see references above), but may also have resulted in a gradual decline in flooding intensity of the former floodplain.

5. Conclusions

In the four sedimentary cases of the subtropical environment in China, a definite fining-upward evolution of floodplain deposition is evident and is characterized by distinct sedimentary facies. The coarse-grained facies which predominates at the bottom of the floodplain depositional series indicates a relatively strong invasion of the floodplain by sheet flows or shallow channels. In contrast, the next fine-grained facies supplied mainly suspended sediments to the floodplain, consisting of reworked loess and very fine sand. After flooding, shallow pools were filled with deposits of clay and very fine silt, ultimately leading to short periods of soil formation. Aeolian dunes occasionally formed on the generally dried floodplain. A gradual sedimentary evolution towards the dominance of fine-grained deposition appears on the abandoned floodplain. It progressed simultaneously with river incision that was initiated after climatic warming. In that sense, both the fluvial processes of incision and flooding of the former floodplain were delayed for some time after the climatic trigger. Short-term influxes of flooding were superposed on top of that long-term evolution.