Channel evolution controlled by valley configuration during 70 years in a severely erosive catchment: Mangaoporo River, New Zealand

Catena 174 (2019) 324–338

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Channel evolution controlled by valley configuration during 70 years in a severely erosive catchment: Mangaoporo River, New Zealand

T

⁎

Mio Kasaia, , Daisuke Aokia, Dave Peacockb, Tomomi Marutanic a

Research Faculty of Agriculture, Hokkaido University, Kita-9 Nishi-9 Kita-ku, Sapporo, Japan Peacock D H Limited, 120 Rosetta Road, Raumati South, Paraparaumu, New Zealand c Hokkaido Research Organization, Kita-19 Nishi-11 Kita-ku, Sapporo, Japan b

A R T I C LE I N FO

A B S T R A C T

Keywords: Aerial photograph interpretation Channel pattern Channel evolution Gully complexes Unit stream power Valley configuration

We examined the channel-evolution process in a 19.5 km course in the Mangaoporo River (catchment area: 72.6 km2) in the East Coast region of North Island, New Zealand over 70 years using aerial photographs and cross-sectional survey records. The catchment experienced expansive deforestation during the late 19th and early 20th centuries, which accelerated the growth of the gully complexes. The manner of the evolution was controlled largely by the underlying lithology that determined the valley configuration. In the upstream confined section, where hillslope and channel processes were closely coupled, continuous sediment supply from gully complexes gradually converted the channel course from a narrow single-thread to wide braided reaches, or from sediment transfer to storage zones. This change was accompanied by a decrease in unit stream power (USP). In 1939, the USP was above 200 W/m2 in most of the upstream section, while 40% of the USP values were near or below 50 W/m2 in 2012/2013. Because a narrow reach situated at the end of the storage zone limited the amount of sediment travelling further, the downstream reaches were gradually disconnected from the upstream section, and the channel forms changed more moderately than in the upstream section. This reduction in sediment travelling with distance transformed a once-braided reach at the end of the course to development of a supply-limited condition with time. The overall channel evolution was accelerated by major rainfall events, including prolonged rainfall in 1956 and Cyclone Bola in 1988, which activated gully complexes and contributed to subsequent channel widening, particularly in the upstream section. Although reforestation starting in the 1980s contributed to deactivation of gully complexes after 1988, the difficulty in controlling their total remission together with the lower USP values of the river in 2012/2013 compared to those before the previous major wet events indicate that the river has been evolving in an irreversible direction, triggered by land use changes of more than a century ago.

1. Introduction During the late 19th and 20th centuries, extensive land conversion from forest to pasture in various parts of the world resulted in increased hillslope erosion, drastic aggradation, and riverbed widening. After more than a century, the impacts of these land use changes are still observable in those catchments, depending on inherent hillslope resilience, and are largely determined by in situ lithology and climate patterns. Some studies have reported that in catchments where hillslopes were stabilized and revegetated following conversion, the upper channels gradually narrowed and degraded, and the growth of channel margins decoupled channel processes from hillslope processes (Marutani et al., 1999). The sediment that accumulated in higher-order

channels during the period became a major sediment source for the entire catchment (Trimble, 1999). In this case, the impact of land conversion will diminish with time and distance (Verstraeten et al., 2017). By contrast, in erodible terrains where hillslopes are entirely destabilized by human disturbance, continuous and direct sediment input from hillslopes into a course keeps aggrading and widening the riverbeds, preventing the growth of channel margins (Gomez et al., 2003). Decreases in sediment transport capacity following changes in channel forms encourage further accumulation of fresh sediment and more widening. The collection of sediment continuously delivered from such growing sediment stores in upstream rivers over a century has increased the risk of flooding in alluvial plains and decreased the ecological value by burying aquatic habitats, as currently seen along the

Abbreviations:USP, unit stream power ⁎ Corresponding author. E-mail address: [email protected] (M. Kasai). https://doi.org/10.1016/j.catena.2018.11.032 Received 22 October 2017; Received in revised form 19 November 2018; Accepted 22 November 2018 0341-8162/ © 2018 Elsevier B.V. All rights reserved.

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Fig. 1. Study site with cross-sectional lines set by the Gisborne District Council in 1958 (yellow lines). Red dotted lines mark the upstream and downstream ends of the study reach. The hillshade map was created from NZSoSDEM v 1.0 (07 East Cape) (https://www.otago.ac.nz/surveying/research/geospatial/otago040574.html). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

margins or floodplains, because of direct sediment input from lateral sources such as landslides and valley walls. Subsequently, sediment is transported according to the local sediment transport capacity. Bagnold (1966) proposed the unit stream power (USP), ω (W/m2) to represent this capacity:

East Coast of North Island, New Zealand (Page et al., 2008), and further landscape changes are expected in the future. Therefore, it is important to understand the characteristics of channel evolution in headwater catchments because land conversion in such erosive environments will have a collective effect on the channel network lasting centuries (Verstraeten et al., 2017). In rivers filled with fresh sediment, valley configuration plays a significant role in determining channel-evolution patterns by controlling the connectivity between hillslopes and channels and channel reaches, and local flow capacity to transport sediment. In confined reaches, channel forms change more sensitively in response to a storm event than downstream reaches running through developed channel

ω = ρ⋅g⋅Q⋅S / W

(1)

where ρ is the fluid density (1000 kg/m ), g is the gravitational acceleration (9.8 m/s2), Q (m3/s) is the flow discharge, and S is the energy slope. Eq. (1) indicates that for the same total stream power, Ω = ρ∙g∙Q∙S, narrow reaches underlain by resistant bedrock tend to work as sediment transfer zones by generating a strong sediment transport 3

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capacity (Magilligan, 1992), while impeding sediment flux at the immediate upstream and controlling the amount of sediment that can travel further downstream (Beschta, 1983). On the other hand, wider reaches located on less resistant bedrock produce a lower transport capacity and are likely to accumulate sediment, to form storage zones (Magilligan, 1992). Because a braided pattern can appear following an increase in sediment input, whereas bars can start developing following its decrease as channel incision becomes dominant (Pryor et al., 2011; Schumm, 1969), USP values are also related to channel patterns (Lewin and Brewer, 2001; van den Berg, 1995) The effects of valley configuration in the upstream reaches would also have an impact on channel evolution in downstream unconfined reaches, through the amount of sediment delivered into these zones. Hence, USP is a key factor for investigation of the characteristics of channel evolution over a channel course and period, and temporal and spatial changes in these values should be examined. To ascertain the sediment supplies from hillslopes and their effects on the channel system after land conversion, historic datasets covering multiple decades are required. Aerial photographs are probably one of the most accessible and useful tools for this purpose. They show chronological changes in both landslides and channel platforms, which indicate increases or decreases in the amount of sediment input before photography. Widened channel beds in a final photography year suggest an increase in the amount of sediment deposits in aggrading rivers (Madej and Ozaki, 1996), and the simultaneous appearance of a braided pattern can support this interpretation. Given a cross-sectional survey conducted between photography years, morphological changes at each survey line can also aid in estimation of the timing of channel erosion and sediment deposition, and indicate the passage of sediment slugs (Nelson and Dubé, 2016). These historic datasets are undoubtedly valuable and significant in examining river behavior in the past; however, they also unavoidably contain some uncertainty, derived from the resolution and the timing of photography for the former, and the locations for the latter (Lea and Legleiter, 2016). To acquire the most reliable scenarios possible, the spatial and temporal connectivity of sediment delivery between geomorphic units, such as hillslopes and channel reaches, should be considered in assembling the available information. For instance, sediment supply from landslides can produce an increase in sediment supply to a channel course, and increase sediment availability or the sensitivity to stream flow, resulting in the release of sediment deposits (Lisle and Church, 2002). Sediment transport capacity also changes with time as rivers are altered by either deposition or erosion of sediment, in terms of their flow depth and energy slope (Pryor et al., 2011). This study explored the channel evolution of the Mangaoporo River, a tributary of the Waiapu River in the East Coast region of North Island, New Zealand (Fig. 1), by examining a sequence of aerial photographs taken from 1939 to 2012/2013 and a cross-sectional survey conducted using six lines starting in 1958. The Mangaoporo River has suffered from continuous sediment supply from gully complexes, the activity of which was accelerated by European immigration in the late 19th and early 20th centuries and has caused continuous aggradation and widening along the course (Page et al., 2008). Reforestation for erosion control was commenced in the late 1980s. The river has always been partially braided at least since 1939, while hard resistant bedrocks constricts valleys in places. This study focused on tracking the changes in the area of bare land, which reflects the growth and waning of gully complexes in response to storm events, and their effects on changes in channel patterns and forms, which gradually propagated downstream, affected by the valley configuration. Temporal and spatial changes in the transport capacity, USP, estimated using a simple method through the study period, help show the direction of evolution of the river. We developed a method for utilizing available datasets that acknowledges their uncertainty, to estimate future channel behavior considering the spatial and temporal patterns of sediment delivery and channel evolution. This approach is applicable to other regions for planning river

control at a catchment scale. 1.1. Study site The East Coast Region of North Island, New Zealand consists of three major catchments: the Waipaoa, Waiapu, and Uawa. The region which occupies just 2.5% of North Island is well known for its severe erosion accelerated by intensive land conversion from forest to pasture in the late 19th and early 20th century (Herzig et al., 2011). It produces 33% of the annual sediment yield from the island (Page et al., 2008). A variety of erosion types, gully complexes, shallow landslides, earth flows and rotational slides, is found in the region, and gully complexes are a major sediment producer (Marden et al., 2014). One of the headwater streams of the Waipaoa River (Fig. 1) aggraded up to 7 m following land conversion, by storing sediment yielded from gully complexes (Gomez et al., 2003). The materials supplied from gully complexes into the channel are highly deformable, and most of them are delivered as suspended load to the alluvial plain (Page et al., 2008). In the lower Waipaoa River they rapidly accumulate in floodplains and levees at rates of at most 60 mm/year (Gomez et al., 1999). In the neighboring Waiapu catchment, > 20% of the area is underlain by Cretaceous-aged sedimentary rocks prone to gully complexes (Page et al., 2008), and the number of active gully complexes is twice that in the Waipaoa catchment (Marden et al., 2005; Page et al., 2008). The catchment yields about 2.5 times the amount of sediment, the largest specific sediment yield (t km2 yr−1) in the world (Walling and Webb, 1996). Sediment from the gully complexes is stored in headwater streams before it is delivered downstream at various rates, which are largely controlled by geologic structure (Tunnicliffe et al., 2018) and drainage network configurations (Walley et al., 2018). In the Waiapu catchment, the rivers used to be narrower and deeper than those today, and hosted healthy freshwater fish communities; however, many parts of these rivers are now braided following an increase in the amount of sediment stored in the courses over the last century (Rosser et al., 2012). To control severe hillslope erosion, reforestation was commenced in the East Coast region from the 1960s to late 1980s (Marden et al., 2011). The model by Herzig et al. (2011) presents the effectiveness of reforestation in reducing the sediment yield, while Marden et al. (2011) commented that some gully complexes in the region were already too large and active at the time of planting to terminate their activity. In their model, gully complexes > 20 ha in size cannot be stabilized within 30 years after forest planting. The growing process of gully complexes in indigenous forest in the Mangaoporo catchment described by Parkner et al. (2007) also demonstrates the difficulty of terminating them completely. Marden et al. (2018) conceptualized those gully complexes as “badass”, a term coined by Phillips (2015), because their growth is driven by a combination of fluvial and mass movement processes, and cannot be explained by any existing gully model. Gomez et al. (2003) indicated that several decades were required until the effects of reforestation appeared in the lower Waipaoa River because a large amount of sediment was still stored in the channels. For the Waiapu catchment with twice the terrain prone to gully complexes, a much longer time will be required to observe the effects even if reforestation is successfully completed. The average annual rainfall for the headwaters of the Waiapu catchment is > 4000 mm/year (Hessell, 1980), and the minimum recurrence interval of a storm event that can induce hillslope erosion is 2.6 years (Hicks, 1995). Cyclone Bola of March in 1988 was the largest storm event since the start of metrological observation in the region, and it produced 900 mm of rainfall over 4 days in the headwater catchments. Fig. 2 shows the maximum 3-day rainfall and total rainfall in a year after 1947 at Te Puia Springs station, 20 km south of Ruatoria, and Ruatoria station (Fig. 1). The rainfall data were obtained from the National Climate Database of NIWA, New Zealand (https://cliflo.niwa. co.nz/). The Te Puia Springs station has been operational for the longest period near the Mangaoporo River. The maximum 3-day rainfall was 326

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Fig. 2. Total annual rainfall and annual maximum 3-day rainfall at Te Puia Springs and Ruatoria gauging stations. The rainfall data were obtained from the National Climate Database of NIWA, New Zealand (https://cliflo.niwa.co.nz/).

unconfined ‘downstream section’ (7.5–19.5 km). A longitudinal profile of the study site was constructed from a 15-m digital elevation model (DEM; NZSoSDEM v 1.0 07 East Cape) created by the National School of Surveying, the University of Otago, New Zealand (https://www.otago. ac.nz/surveying/research/geospatial/otago040574.html); this is presented in Fig. 4. Although the elevation values were not strictly accurate (Columbus et al., 2011), the data source was considered sufficiently reliable to provide an approximate value for the study site.

used here as a representative of the intensity of the largest storm event in a year. While storm intensity differed among locations for some years, the rainfall pattern at Ruatoria mostly coincided with the one at Te Puia Springs for the period during which data were available. The area of the Mangaoporo catchment is 72.6 km2, and the elevation ranges from 40 to 1413 m a.s.l. (Fig. 1). The total area of gully complexes has increased since 1939, reaching 263.4 ha in 1957 (Rosser et al., 2012). After 1957, shrubs gradually invaded and covered gully surfaces, shrinking until their reactivation by Cyclone Bola in March 1988. After the cyclone, gully surfaces were gradually revegetated. The total area of the gully complexes in the catchment decreased from 391.5 ha in 1997 to 309.2 ha in 2008 (Rosser et al., 2012). Reforestation by planting the fast-growing tree Pinus radiata began in the 1980s, and forested areas doubled to 64% between 1969 and 2008 (Rosser et al., 2012). A 19.5 km segment of the Mangaoporo River was selected as the study site. The downstream end was situated at a bridge 2 km upstream of the confluence with the Waiapu River. The segment contained four reaches wider than their neighbors: W1 (2–6 km from the upstream end), W2 (7.5–10 km), W3 (10.5–12.5 km), and W4 (16.5–19.5 km), three narrow reaches between these: N1 (7–7.5 km), N2 (10–10.5 km), and N3 (15.5–16.5 km), and one that we called B, which indicates the bend (6–7 km) (Figs. 1 and 3). The N1 reach was laterally confined by the Late Cretaceous Tikihore and Tapuaeroa formations (Mazengarb and Speden, 2000; Parkner et al., 2007), and hillslope and fluvial processes were closely coupled. Downstream from 7.5 km, the river runs through alluvial deposits, except for the N2 reach, which is underlain by the Tapuaeroa formation (Mazengarb and Speden, 2000; Parkner et al., 2007). The study course was divided into two sections: the mostly confined ‘upstream section’ (0–7.5 km), and the mostly

2. Methods Changes in the area of bare land on hillslopes and channel platforms were tracked with sets of aerial photographs taken in November 1939, April 1957, August 1971, June 1988, and March 1997, and with orthophotographic images taken during 2012 and 2013 obtained from the Land Information New Zealand website (2017; https://data.linz.govt.nz/Layer/1722-gisborne-04m-rural-aerial-photos2012–2013/). Bare land and riverine areas were identified as ground surfaces with little vegetation growth and outlined on the 2012/2013 image using the ArcGIS software. For other years, these areas were traced onto the aerial photographs using stereoscopy. Figures obtained from the interpretation of aerial photographs were digitally scanned and rectified using the 2012/2013 image within ArcGIS. Because gully complexes were occasionally difficult to outline due to partial shading and light reflection on the images, the area of bare land was instead measured for comparisons between years, assuming that a large portion of them contain gully complexes. In the cases where the image of a year did not cover the entire catchment, the area was estimated from the photographs before and after the year. Thus, the area presented in Fig. 5 is rather referential, although it is not far from the total area of 327

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Fig. 3. Geological map of the Mangaoporo River catchment, based on Mazengarb and Speden (2000) and Parkner et al. (2007). Kri A: Tikihore Formation, Early Cretaceous; Kri B: Tikihore Formation, Late Cretaceous; Kri C: sandstone dominated by Tikihore Formation, Late Cretaceous; Krp: Tapuaweroa Formation, Late Cretaceous; IQa: undifferentiated alluvial deposits, usually deeply weathered, Late Quaternary; Kiw: Whangai Formation, Early Tertiary; Egw: Wanstead Formation, Early Tertiary; Q1a: poorly sorted fan gravels overlain by Tephra, Late Quaternary. Extents of reaches are separated based on valley widths (shown).

gully complexes shown in the previous section. Because the rates of erosion due to landslides can be indirectly estimated from their areas (Derose et al., 1998; Guzzetti et al., 2009), it was considered that changes in bare land area indicated the increase or decrease in the amount of sediment supply from the external sediment source into the main channel course. Similarly, some parts of the river course were shaded by riparian trees in 1957 and 1988, which made it difficult to identify bank edges. However, the affected reaches were limited and we assumed that they did not significantly affect the tracking of geomorphological changes during the study period. The changes in channel patterns and forms were obtained using the images above and six cross-sectional lines, CS8420, CS10320, CS13100, CS16490, CS18680, CS19500, set up from 8 to 19.5 km in 1958 by the Gisborne District Council (Fig. 1). The number of lines denotes the distance (m) from the upstream end of the study course. The mean bed elevation over the active channel width was employed here to find the timing of channel bed aggradation and degradation. Channel patterns were categorized into four types or ‘states’ (Fig. 6), based on the development of bars, terraces, and channel courses. A 500 m long segment was used as the unit of a channel reach in this study. The average channel width for a reach, W (m), was obtained by dividing the area of a reach polygon by 500, after splitting the polygon for the entire riverine area into 500 m segments along the study site. The mean bed levels at each cross-section were obtained every 1–2 years, based on the surveys. The surveys were terminated in 1998 for the cross-sectional line that was farthest upstream (CS8420), due to destruction of the

Fig. 5. Change in bareland area for the upstream and downstream sections. Asterisk: estimated areas from the aerial photographs before or/and after the year (presented with shading).

benchmark and a tributary fan that grew on the CS10320 line. The USP was calculated using Eq. (1) for each 500 m reach. This study employed the channel slope obtained from the profile shown in Fig. 4 as S, under the assumption that the elevation value error and change in channel slope are negligible at this spatial scale over the study period. The mean bed levels of the six cross-sections were aggraded by 1.4 m from the maximum between 1958 and 2015. The mean annual flood discharge, Qa, was applied for Q, and Qa for each 500 m reach was estimated from the relationship between catchment area, A (km2), and Qa, of four gauging stations in the Waipaoa catchment and one from Waiapu catchment, presented in the literature (Hicks et al., 2000). The relationship is given by:

Fig. 4. Longitudinal change in channel slope and the location of cross-sectional lines. The extents of reaches are also presented. Channel slope was calculated from NZSoSDEM v 1.0 (07 East Cape). 328

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Fig. 6. Channel categorization based on the planform pattern. Wma: width of main active channel; Wa: width of other active channels.

Qa, =5.0472A0.7887 (R2 = 0.974)

(2)

where A for each reach was measured at the middle point of the reach. Because of the limited availability of the data necessary for Eq. (2), it was presumed that the given value of Qa is approximate. Nevertheless, a reasonably high coefficient of determination suggests that the order of the value should not largely differ from reality. 3. Results 3.1. USP values and channel patterns The USP values for each channel pattern appearing in a reach throughout the study period are shown in Fig. 7. The reaches containing mixed states were removed from the plots. The values for States 1 and 4 were distinctly separate, while the ranges of the State 2 and 3 patterns were almost identical and plotted between the other states. Based on the boxplots, State 1 likely appeared in reaches with USP values exceeding 200 W/m2, while State 4 prominently appeared in reaches where the value was below 50 W/m2. Given that the bed median grain size sampled at 323 points along the study course in 2004 ranged from 14 to 140 mm (Fig. 8, unpublished data by Tsuchiya), the USP range for State 4 fits the relationship between the grain size and USP at bankful discharge for braided channels presented by Lewin and Brewer (2001). The range for State 1 was also compatible with the relationship between the same factors presented by van den Berg (1995) for straight and single-thread channels. 3.2. Chronological changes in channel forms and USP in response to sediment yielded from hillslopes

Fig. 7. Box plots of unit stream power for each state. The interquartile range is represented by the box. Upper and lower whiskers are the maximum and minimum values respectively. Outliners are also plotted. The horizontal line represents the median unit stream power and n is the number of plots.

3.2.1. November 1939 In 1939, the area of bare land on the aerial photographs was smallest during the study period (Figs. 5 and 9). The upstream section was narrow and the spatial variation in the width along the course was 329

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Fig. 8. Downstream variation in D50 (median diameter of surface bed materials). The data were collected by Tsuchiya in 2004 (unpublished). Because the exact sampling sites are unknown, the locations along the course may not be accurately plotted.

upstream section (Figs. 9 and 12). The proportion of State 1 in the section remained the same from 1939, while the locations changed (Fig. 12). Nonetheless, expansive channel widening was observed along the site from 1 to 10 km, interfered by the N1 reach (Fig. 10b). As a result, the channel width in the W1 reach became similar to that of the W3 reach. Simultaneously, the B reach became wider than the N1 reach (Fig. 10a). The rapid increase in the mean bed level of CS8420 (8.42 km) suggests a consistent sediment supply into the W2 reach from the upstream section (Fig. 13). By contrast, the mean bed level of CS10320 (10.32 km) increased at a slower rate, which indicates that the reach acted as a sediment buffer. Judging from the appearance of the State 2 pattern in the upper part of the reach and State 4 in the lower part, the spatial pattern of channel widening, and the slowing of aggradation at CS8420 after 1968 (Figs. 9, 10b, and 13), a large amount of sediment as a sediment slug could have passed the line (CS8420) and stayed in the W2 reach, otherwise moving to the upper half of the W3 reach by 1971. Further downstream of the lower half of the W3 reach, a change in channel width was observed to a much smaller degree than in the upstream reaches (Fig. 10b). At the cross-sections of CS13100, channel aggradation occurred in the early 1960s and then degraded until 1968 (Fig. 13). In turn, channel incision occurred in the early 1960s and then aggraded at the CS16490, CS18680, and CS19500 lines. A change in the pattern in the narrowed reach, 13–14.5 km, from State 3 to 2, and the N3 reach from State 1 to 2 during the period (Figs. 9 and 10b) together imply that sediment was released from the former reach and trapped in the latter reach.

small compared to the latter years (Fig. 10a). The USP of 85% of the section exceeded 200 W/m2 (Fig. 11). The pattern of State 1 appeared dominant downstream to 12 km (Fig. 9), along which terrace deposits covered with grass were observed in the aerial photographs. The exceptions were upstream of the B and N2 reaches, which were choked with sediment (Fig. 9); State 4 appeared in the latter. Downstream from 12 km, reaches were wider and gentler than the upstream section (Figs. 4 and 10a). The prominent appearance of States 3 and 4 implies that the reach trapped sediment efficiently. Assuming that the missing part of the image also showed State 4, this was the only time that channel braiding extended as far as 17.3 km. 3.2.2. November 1939–April 1957 Gully complexes activated by 1957 (Fig. 9) induced channel widening along the course (Fig. 10b). No notable storm events occurred between 1946 and 1957, while the largest annual rainfall was recorded in 1956 (Fig. 2). Hence, this activation was considered to have been derived from wet weather, perhaps prolonged moderate rainfall, in 1956. The headwater reaches, from 0 to 3 km, still maintained the dominant appearance of State 1 and a USP above 200 W/m2 during the period, while > 60% of reach W1 exhibited a decreased USP of below 200 W/m2, which indicates that it had gradually turned into a transitional reach from a single threaded reach (Figs. 9 and 11). Increase in sediment supply from hillslopes induced a State 4 pattern from 3 to 3.5 km in the reach, with a USP consistently below 50 W/m2, and from 4 to 4.5 km. The 1957 aerial photographs showed that sediment traveled from the upstream section through the narrow B and N1 reaches into the upper part of the W2 reach and buried 1939 tributary fans and terraces to induce a State 4 pattern (Fig. 9). Further downstream, State 3 dominated to 15.5 km, while a State 1 pattern remained in the N3 reach with a USP above 200 W/m2 and little change in the channel width since 1939 (Figs. 9, 10b, and 11). Noticeable channel widening and the appearance of States 3 and 4 in the W4 reach indicate that sediment travelling through reach N3 was deposited there (Figs. 9 and 10b).

3.2.4. April 1971–June 1988 In March 1988, Cyclone Bola increased the area of bare land by activating gully complexes, particularly in the upper catchment, increasing the riverine areas occupied by a State 3 pattern in the upstream section, although, interestingly, no State 4 pattern was observed in this section (Figs. 5, 9, and 12). In fact, the aerial photographs showed that the channel had already been incising newly deposited materials. A State 1 pattern appeared in the reaches from 0 to 0.5 km, with a USP above 200 W/m2, and in the upper half of the B reach (Figs. 9 and 11). This could be because sufficient water was still provided to flush out or incise the newly deposited sediment at the later stage of such an extreme event. The channel widened noticeably from the top end to the W3 reach, decreasing the USP values to almost below 200 W/m2 through the course. As a result, the USP of the upstream section became similar to or even smaller than the downstream section (Figs. 10b and 11). The USP value in the upstream part of the B reach

3.2.3. April 1957–August 1971 The amount of bare land shrank between 1957 and 1971 (Figs. 5 and 9). This period had a lack of storm events exceeding 300 mm/ 3 days, although it was not characteristically dry compared to others (Fig. 2). In response to the decline of sediment supply from hillslopes (Figs. 5 and 9), the riverine area presenting a State 4 pattern disappeared and the areas representing a State 3 pattern decreased in the 330

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1971. Still, sediment probably traveled rapidly through the site and accumulated in the W4 reach, where a State 4 pattern was evident (Fig. 9) and the mean bed levels at CS18680 and CS19500 increased in 1988 (Fig. 13). 3.2.5. June 1988–March 1997 During this period, the bare land area decreased only slightly and the upstream section continuously received sediment yielded from this area (Figs. 5 and 9). Narrowed reaches with the State 1 pattern and widened reaches with the State 3 pattern appeared alternately in the upstream section (Figs. 9 and 10b). The channel course from 0 to 4.5 km widened, decreasing the USP values to below 200 W/m2 for the first time during the study period (Figs. 9, 10b, and 11). The N1 reach also exhibited a reduced value to below 50 W/m2 (Fig. 10b) due to lateral erosion into old terrace deposits as observed in the aerial photographs. The appearance of the State 4 pattern in the upstream part of the W2 reach was accompanied by an increase in the mean bed level at CS8420 in the mid-1990s (Figs. 9 and 13). This was the only section presenting this state throughout the entire course in 1997. In the N2 and W3 reaches, the channel widened and the USP values decreased, while the State 1 and 2 patterns were presented. For the rest of the reach down to 800 m from the downstream end, the appearance of the patterns did not change from 1988. The mean bed levels at CS16490 and CS13100 maintained the same fluctuating values over the period (Fig. 13). Downstream from 18 km, the channel narrowed, and for the first time during the study period, a State 4 pattern did not appear in the downstream end. The mean bed level continued to drop following Cyclone Bola until 1998 at CS18680, and the level at CS19500 was nearly stable until at least 1994 (Fig. 13). The overall changes in channel forms and patterns in the downstream section suggest that the channel course could be in a supply-limited condition. 3.2.6. March 1997–2012/2013 The bare land area in the upper catchment waned by 2012/2013 despite an increase in the magnitude of rainfall events after 2003, while this area was larger still than before Cyclone Bola (Figs. 2, 5 and 9). The State 1 pattern from 0 to 0.5 km was accompanied by channel narrowing from 0 to 1.5 km, which occurred for the first time during the study period (Figs. 9 and 10b). It was expected that the reach from 0.5 to 1.5 km would experience a conversion of the pattern to State 1, as the sediment supply from the adjacent bare land decreased. In fact, the Google Earth image for January 2017 demonstrated that channel incision was progressing along the reach with gully complexes deactivated and revegetated. For the period from 1997 to 2012/2013, a moderate increase in channel width suggests that sediment was deposited in the downstream reach from 4 to 7.3 km, a part of the W1 reach to the N1 reach. The channel course exhibited a State 4 pattern in places, with a USP below or around 50 W/m2 (Figs. 9, 10b and 11). This pattern had already appeared in the Google Earth image for December 2002. A total of 40% of the upstream section by length was braided in 2012 (Fig. 12). In the downstream section, a State 2 pattern appeared in the upper part of the W2 reach, while in the lower part the aerial photographs showed that bars formed in the previous period were buried in the latter period, and the State 4 pattern was observed (Fig. 9). The pattern could already be recognized in the Google Earth image for December 2002. Downstream from the W2 reach to 14 km, including the N2 and W3 reaches, the pattern changed from States 1 and 2 to 3 between 1997 and 2012/ 2013. This change became evident in the Google Earth image for January 2013. While State 1 appeared from 14 to 16.5 km with channel narrowing, the increase of the mean bed level from 2003 to 2007 at CS13100 with little subsequent change, and after 2007 at CS16490 (Fig. 13), implies that sediment continued to travel through these reaches. Further downstream, the State 2 pattern appeared for the first time in the entire study period from 16 to 19.3 km in the W4 reach. The Google Earth image for September 2009 demonstrated that the pattern had started to appear in the reach, while the mean bed level at CS18680

Fig. 9. Change in spatial arrangement of the channel pattern. Aerial photographs were unavailable for missing areas. Bare land area includes gully complexes, which form massive landslides features, as well as other landslides (e.g., shallow landslides, earth flows) and tributary channels. Purple lines indicate cross-section lines set by Gisborne District Council. The dotted broken lines for CS8420 and CS10320 in 2012 indicate that the survey data were no longer available. The extent of the reaches is also presented. The large triangles denote each 5 km mark, while small triangles indicate every 1 km mark. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

decreased down to 50 W/m2, while the downstream part reduced to a value indicating it was likely to be a transitional reach. The role of the N1 reach also transformed from sediment transfer (State 1) to storage (State 3), bringing the USP down to almost 50 W/m2 (Figs. 9 and 11). This trend continued until at least 2012/2013, judging from the patterns (Fig. 9). In the downstream section, the W2 reach was entirely occupied by the State 4 pattern (Fig. 9). The mean bed levels of the cross-sections at CS8420 and CS10320 both aggraded, although minor effects on the former suggest that a large amount of sediment such as a sediment slug rapidly traveled through the upstream end of the W2 reach (Fig. 13). The appearance of the State 3 pattern down to 12.5 km and a rapid increase in the mean bed level at CS13100 and a decrease in the bed level at CS16490 in 1988 (Figs. 9 and 13) may indicate that the majority of the slug stopped at some point between the lines. However, this interpretation could be biased because of the bridge located at the upstream of CS13100, which probably induced local channel widening from 12 to 12.5 km by being plugged with sediment. In the downstream reach from 13 km, the channel pattern did not change significantly from 331

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a

Fig. 10. (a) Active channel width along the course and its change after 1939. (b) Annual rates of channel width change between years photographs taken. The reaches where aerial photographs were unavailable are shown as N/A. The locations of cross-section lines and the extent of the reaches are also presented.

4. Discussion

had changed little since 1997. Considering that the USP values were below or near 50 W/m2 in the reach (Figs. 9 and 11), this pattern suggests that the channel became more supply limited than in the previous period, although an amount of sediment capable of maintaining the elevation was continuously delivered. In other words, the entire W4 reach still had the potential to be braided, once a plentiful supply of sediment was delivered and accumulated, as the pattern appeared at the downstream end with channel bed aggradation at CS19500 for the period (Fig. 9). This accumulation of sediment could be caused by the bridge near the cross-section line.

4.1. Channel evolution in the Mangaoporo River during the 70 years This study examined changes in sediment yield from hillslopes and channel forms in the Mangaoporo River over a period of 70 years, to reveal the history of the growth of the upstream section and the W2 reach as a major sediment store, and of a supply-limited condition in the W4 reach. Finer bed material size in the upstream section than the downstream along the study course, which is opposite from the general 332

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b

Fig. 10. (continued)

continuous sediment supply and sufficient sediment transport capacity (Leopold and Wolman, 1957). The state was also present over a longer distance than in 1957, which indicates that there was more sediment stored in the course. As the accumulated sediment kept moving downstream in the course presenting initially high USP values, the role of the B and N1 reaches working as a valve to adjust the sediment flux diminished in turn, evident by 1971 for the former and by Cyclone Bola for the latter. That is, the reaches in the upstream section and the W2 reach were connected more closely in terms of sediment flux with time. A large portion of sediment delivered into the W2 reach, however, was buffered due to its persistently low transport capacity, as indicated by the morphological change in the cross-section lines and the constant appearance of the State 4 pattern. As such, the N2 reach maintained a role as a valve to reduce the amount of sediment travelling further downstream, resulting in very slow transformation of the W3 reach from the single-threaded to transitional states and inducing little change in the USP value from 12.5 km to the downstream end during

trend of rivers (Fig. 8), also supports this interpretation. The evolution of the upstream section was largely due to direct and continuous sediment input from bare land into the channel, which led to channel widening and a decrease in sediment transport capacity to the system. The USP of 80% of the section exceeded 200 W/m2 in 1939, while the value was near or < 50 W/m2 in 40% of the course in 2012 (Fig. 11). The process was accelerated by wet weather in 1956 and Cyclone Bola in 1988. Although there are no historical vertical data available, an increase in the amount of sediment stored in the channel course with time was evident from the appearance of the patterns in each photography year. For example, despite an almost identical estimated bare land area on the hillslopes in 1939 and 1971, the area presenting State 1 was much smaller in 1971. The appearance of State 4 in 2012 in the section for the first time since 1957 could also be attributed to the increase in high magnitude rainfall events after 2003 and the amount of sediment yielded from bare land that continued to accumulate in the course, considering that the formation of braided rivers requires a 333

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Fig. 11. Unit stream power along the course and changes after 1939. The reaches where aerial photographs were unavailable are shown as N/A. The locations of cross-section lines and the extent of the reaches are also presented.

event orientates a long-term evolution of the entire river course, by receiving sediment directly from lateral sources and holding them depending on a given transport capacity. The significance of valley configuration on channel-evolution process could be highlighted by comparing the study from the neighboring Tapuaeroa River (Fig. 1), which runs through Holocene alluvial deposits (Mazengarb and Speden, 2000). Tunnicliffe et al. (2018) organized cross-section survey data starting in the 1950s and showed that Cyclone Bola activated gully

the study period (Fig. 11). That is, the W2 and N2 reaches together evolved as a natural sediment retention dam, by disconnecting their upstream and downstream reaches and with the latter reaches being insensitive to change in response to a series of storm events. It has been widely regarded that valley configuration is a major controller of channel behavior in an intense flood (Fuller, 2008; Thompson and Croke, 2013). This study also described that in erosive environments the behavior of confined channels during a major storm

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Fig. 12. Change in the proportion of the river length for each state. Unidentified reaches due to a lack of aerial images are indicated as missing.

contributed to the growth of channel margins to disconnect the effects of hillslope processes in tributaries, and the channels had developed as sediment sinks to limit the downstream movement of sediment. If gully complexes can be successfully stabilized under the reforestation program, a similar negative feedback process may be observed in the upstream section of Mangaoporo River, and the entire study course would return to 1939 conditions. That is, if the active channel width becomes narrower than the valley width in the N1, N2, and N3 reaches, they cannot work as valves to control sediment delivery downstream any longer, and sediment, the amount of which is much less than today, could be transported directly to the W4 reach to introduce a braided pattern. However, the river could be evolving in an irreversible way already. According to the gully complex model by Herzig et al. (2011), if all gullies in the Waiapu catchment had been forested by 2010, the sediment yield could have been halved from 22 million to 11 million tons/year by 2050. Applying this scenario, which is not the reality, and simply halving the bare land area in 2012/2013, the hillslope condition in 2050 could be similar to that in 1939 or 1971 (Fig. 5). However, the appearance of the channel patterns differed between those years because the transport capacity decreased with time (Fig. 9), probably as in 2050. With lower USP values in 2012/2013 than in 1939 and 1971 (Fig. 11), and the difficulty of stabilizing gully complexes > 20 ha in size by tree planting (Marden et al., 2011), it is still possible that an extreme storm or prolonged wet weather could reactivate gully complexes to develop the upstream section into more definite storage reaches to bring down the USP down to 50 W/m2. The W2 reach is underlain by deeply weathered alluvial deposits and may aggrade and widen further by receiving sediment from the upstream reaches to decrease the USP, so that the role as a major sediment buffer can be maintained. A reduction in the sediment travelling in the reach would

complexes in the catchment, and subsequently the bed rapidly accumulated sediment until around 2010. Then the volume of sediment stored in the river decreased, possibly suggesting that the impact of Cyclone Bola had already started to subside. On the other hand, the channel pattern and USP in the Mangaoporo River imply that the impact on the channel course remained in the upstream section longer, and had not reached the downstream by 2012/2013. The manner of response to Cyclone Bola of the Tapuaeroa River presented by Tunnicliffe et al. (2018) indicates a much shorter residence time of sediment deposits than the Mangaoporo River, which is attributable not only to its larger catchment area (330 km2) but also to a lack of confined reaches to buffer sediment in the upstream reaches. In the future, the direction and rate of channel evolution in the Mangaoporo River will depend on reactivation of gully complexes or the progress of reforestation, given the condition that climate does not change largely. In the study region, an extensive reforestation program has been undertaken, with a series of field studies and models reporting that it has successfully reduced hillslope vulnerability (Derose et al., 1998; Herzig et al., 2011; Marden et al., 1992, 2014; Phillips et al., 2018). Kasai et al. (2005) demonstrated that shallow landslides became the major sediment sources, rather than gully complexes during Cyclone Bola, and the rapid decrease in sediment transported after the event induced a narrow, degrading channel in one of the tributary catchments of the Mangaoporo River (Weraamaia stream, Fig. 1). The two headwater catchments in the Waipaoa River experienced similar changes in channel forms following the remission of gully complexes (Marutani et al., 1999). This study also stated that channel narrowing and incision had already started in the headwater reaches in 2012/ 2013, and the trend continued even up to 2017. Trimble (1999) showed that a decrease in sediment from hillslopes as well as in flow discharge 335

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other hand, in the worst case scenario applied to the model by Herzig et al. (2011), in which afforestation efforts in the Waiapu catchment are terminated, the sediment yield would be doubled to 44 million t a−1 by 2050. If this occurs, the hillslope would produce more sediment than that induced by Cyclone Bola. In this case, similar processes of channel change as seen in rivers left with an enormous amount of mining waste, or those recently exposed to extensive land conversion, which eventually changed the valley configuration with the migration of a “super slug”, may be observed (James, 1991; Knighton, 1989; Moody, 2016; Nicholas et al., 1995; Sims and Rutherfurd, 2017); i.e., a rapid increase in sediment availability with a surplus of sediment in the upstream section may cause a strong transport-limited condition (Lisle and Church, 2002), resulting in an increase in the amount of sediment stored and then delivered through the entire network. While these are just imaginary scenarios, they still demonstrate the benefit of reforestation for mitigating the impact of land conversion, which would be reflected in the Waiapu River in the years to come. 4.2. Uncertainties and future research This study demonstrated that a series of aerial photographs and cross-section data are very useful for understanding the long-term trends of channel evolution in response to hillslope erosion. Our findings derived from the data set are crucial for avoiding potential misinterpretations that could result from short-term observations from limited locations. Further investigation is challenging due to uncertainties that should be addressed. For example, although the USP value was a significant factor for assessing the intrinsic flow capacity of channel reaches at each time slice, and the value ranges were apparently satisfied by the existing relationship between the USP and the median grain size (Lewin and Brewer, 2001; van den Berg, 1995), there are a number of issues to improve and develop the method further. First, the elevation data obtained from open source information are not strictly accurate. Second, our assumption that the longitudinal change is negligible in calculating the bed slope is based on the elevation data of six cross-sections in the downstream section, and may not be applicable in the upstream section in which channel forms have changed more dynamically. Third, a limited number of flow gauges in the Waiapu catchment forced us to use flow data from the neighboring catchment system, the Waipaoa River. The period of data sampling also varied between stations. Thus, the relationship between catchment area and flow discharge could differ from the reality. Fourth, the riverine area required for obtaining the active channel width was not always accurate, due to the shade created by riparian forests and valley walls in the images. The USP value was linked with the channel patterns in this study, although the patterns may not be deterministic either, as the aerial photograph images used were taken at various flow stages, which affected recognition of the channel courses. In addition, although the cross-section surveys provided valuable information on channel processes and helped to underpin the interpretation of aerial photographs, their changes were not strictly representative of the entire channel course due to the separation of several kilometers between the crosssection lines. The lines were also set in relatively approachable locations, such as near a bridge, where morphological changes were likely to be affected by artificial structures. While the results indicated that some morphologic changes in the Mangaoporo River were probably associated with the migration of sediment slugs initiated by gully complexes, this cannot be confirmed solely from the available data sets. However, in reality, data shortages are common in investigating geomorphic processes in the past. Our findings will be corrected and strengthened by accumulating geomorphic and hydrological data sets, which are detailed, accurate, and frequently measured, with the help of modern technology, such as high-resolution satellite images and ground elevation data from LiDAR surveys by manned and unmanned air vehicles, and increasing numbers of flow gauging sites. The comparison between the Mangaoporo River and the Tapuaeroa River brings us back

Fig. 13. Change in mean bed level at cross-sectional lines surveyed by the Gisborne District Council. Arrows indicate the years when aerial images were available.

prevent the N2 reach from turning into a sediment store, as happened in the N1 reach in 1988. Still, as long as there is fresh sediment available in the upstream section and W2 reach with a continuous sediment supply from gully complexes, which do not recover for years after being activated, sediment will keep flowing into the W3 reach to contribute to gradual widening of the channel course and evolution towards a braided reach in the very distant future. Sediment supply from growing storage zones would also work just to maintain the channel bed elevation in the downstream section, as seen in recent history. From this perspective, the W4 reach could be considered in the equilibrium condition, while the USP value would keep implying that it is in the supplylimited condition demonstrating the potential to be braided. Overall, the river would keep following the current evolutionary path. On the 336

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to the issue of a “black box” discussed by (Walling, 1983) in terms of catchment sediment cascades (i.e., spatial and temporal changes in sediment stored along the channel course must be considered in estimating the catchment sediment delivery ratio). By enabling estimation of the amount of sediment supply from hillslopes and the manner of the channel response more accurately and frequently, the balance between the sediment supply and transport capacity, and sediment residence time in each storage zone can be more confidently evaluated. This means the ‘black boxes’ can be investigated more quantitatively, to help develop appropriate and efficient river management practices over decades, or even centuries.

elevation model for New Zealand. Surv. Q. 66, 16–19. van den Berg, J.H., 1995. Prediction of alluvial channel pattern of perennial rivers. Geomorphology 12, 259–279. https://doi.org/10.1016/0169-555X(95)00014-V. Derose, R.C., Gomez, B., Marden, M., Trustrum, N.A., 1998. Gully erosion in Mangatu forest, New Zealand, estimated from digital elevation models. Earth Surf. Process. Landf. 23, 1045–1053. https://doi.org/10.1002/(SICI)1096-9837(1998110) 23:11<1045::AID-ESP920>3.0.CO;2-T. Fuller, I.C., 2008. Geomorphic impacts of a 100-year flood: Kiwitea Stream, Manawatu catchment, New Zealand. Geomorphology 98, 84–95. https://doi.org/10.1016/j. geomorph.2007.02.026. Gomez, B., Eden, D.N., Hicks, D.M., Trustrum, N. a, Peacock, D.H., Wilmshurst, J., 1999. Contribution of floodplain sequestration to the sediment budget of the Waipaoa River, New Zealand. Geol. Soc. Lond. Spec. Publ. 163, 69–88. https://doi.org/10. 1144/GSL.SP.1999.163.01.06. Gomez, B., Banbury, K., Marden, M., Trustrum, N.A., Peacock, D.H., Hoskin, P.J., 2003. Gully erosion and sediment production: Te Weraroa Stream, New Zealand. Water Resour. Res. https://doi.org/10.1029/2002WR001342. Guzzetti, F., Ardizzone, F., Cardinali, M., Rossi, M., Valigi, D., 2009. Landslide volumes and landslide mobilization rates in Umbria, central Italy. Earth Planet. Sci. Lett. 279, 222–229. https://doi.org/10.1016/j.epsl.2009.01.005. Herzig, A., Dymond, J.R., Marden, M., 2011. A gully-complex model for assessing gully stabilisation strategies. Geomorphology 133, 23–33. https://doi.org/10.1016/j. geomorph.2011.06.012. Hessell, J.W.S., 1980. The climate and weather of the Gisborne region. New Zeal. Meteorol. Serv. Misc. Publ. 115, 1–30. Hicks, D.L., 1995. A way to estimate the frequency of rainfall-induced mass movements. J. Hydrol. 33, 59–67. Hicks, D.M., Gomez, B., Trustrum, N.A., 2000. Erosion thresholds and suspended sediment yields, Waipaoa River Basin, New Zealand. Water Resour. Res. https://doi.org/ 10.1029/1999WR900340. James, L.A., 1991. Incision and morphologic evolution of an alluvial channel recovering from hydraulic mining sediment. Geol. Soc. Am. Bull. 103, 723–736. https://doi.org/ 10.1130/0016-7606(1991)103<0723:IAMEOA>2.3.CO;2. Kasai, M., Brierley, G.J., Page, M.J., Marutani, T., Trustrum, N.A., 2005. Impacts of land use change on patterns of sediment flux in Weraamaia catchment, New Zealand. Catena 64, 27–60. https://doi.org/10.1016/j.catena.2005.06.014. Knighton, A.D., 1989. River adjustment to changes in sediment load: the effects of tin mining on the Ringarooma River, Tasmania, 1875–1984. Earth Surf. Process. Landf. 14, 333–359. https://doi.org/10.1002/esp.3290140408. Lea, D.M., Legleiter, C.J., 2016. Mapping spatial patterns of stream power and channel change along a gravel-bed river in northern Yellowstone. Geomorphology 252, 66–79. https://doi.org/10.1016/j.geomorph.2015.05.033. Leopold, L.B., Wolman, M.G., 1957. 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In: Henriques, P.R. (Ed.), The Proceedings of the International Conference on Sustainable Land Management. Napier, New Zealand, pp. 358–361. Marden, M., Arnold, G., Gomez, B., Rowan, D., 2005. Pre- and post-reforestation gully development in Mangatu Forest, East Coast, North Island, New Zealand. River Res. Appl. 21, 757–771. https://doi.org/10.1002/rra.882. Marden, M., Herzig, A., Arnold, G., 2011. Gully degradation, stabilisation and effectiveness of reforestation in reducing gully-derived sediment, East Coast region, North Island, New Zealand. J. Hydrol. New Zeal. 50, 19–36. Marden, M., Herzig, A., Basher, L., 2014. Erosion process contribution to sediment yield before and after the establishment of exotic forest: Waipaoa catchment, New Zealand. Geomorphology 226, 162–174. https://doi.org/10.1016/j.geomorph.2014.08.007. Marden, M., Fuller, I.C., Herzig, A., Betts, H.D., 2018. Badass gullies: Fluvio-massmovement gully complexes in New Zealand's East Coast region, and potential for remediation. Geomorphology 307, 12–23. https://doi.org/10.1016/j.geomorph. 2017.11.012. Marutani, T., Kasai, M., Reid, L.M., Trustrum, N.A., 1999. Influence of storm-related sediment storage on the sediment delivery from tributary catchments in the Upper Waipaoa River, New Zealand. Earth Surf. Process. Landf. 24, 881–896. https://doi. org/10.1002/(SICI)1096-9837(199909)24:10<881::AID-ESP17>3.0.CO;2-I. Mazengarb, C., Speden, I.G., 2000. Geology of the Raukumara area. Inst. Geol. Nucl. Sci. 1250,000 Geol. Map. doi: 10.1017/CBO9781107415324.004. Moody, J.A., 2016. Residence times and alluvial architecture of a sediment superslug in response to different flow regimes. Geomorphology 294, 40–57. https://doi.org/10. 1016/j.geomorph.2017.04.012. Nelson, A., Dubé, K., 2016. Channel response to an extreme flood and sediment pulse in a mixed bedrock and gravel-bed river. Earth Surf. Process. 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5. Conclusions This study tracked landscape changes in the Mangaoporo catchment in the East Coast region of North Island, New Zealand, during 70 years using a series of aerial photographs taken from 1939 onward and crosssectional survey data from 1958 onward. Channel evolution in the Mangaoporo River was strongly controlled by valley configuration, or the underlying lithology. The history can be summarized as growth of sediment storage zones in the upstream section, accompanied by increases in the connectivity between reaches within it, followed by the development of supply-limited conditions in the channel at the downstream end, resulting from the progressive disconnection between the upper confined and lower unconfined reaches. The W2 reach, an evermajor sediment storage reach, and the N2 reach, located in the middle of the study course were largely responsible for this disconnection. The evolutionary process was accelerated by wet weather in 1956 and Cyclone Bola in 1988 that brought a large amount of sediment into the channel course, triggering a subsequent decrease in USP in the upstream section with the accumulation of stored sediment and contributing to further development of sediment storage zones to lengthen residence time. After Cyclone Bola, the bare land area decreased and channel incision and narrowing commenced in the most upstream reach, which suggests that erosion control by reforestation starting in 1980 appeared to be working. However, lower USP values in 2012/ 2013 than before the cyclone and the difficulty of complete termination of gully complexes suggested by models (Herzig et al., 2011; Marden et al., 2011) still indicate the possibility that the river keeps following the evolutionary path set over the past 70 years. That is, the river could already be evolving in an irreversible way. This study used a series of aerial photographs and cross-section data as well as open source elevation data to understand the long-term trend of channel evolution in response to hillslope erosion. Uncertainties in the data and the analyses may obscure our interpretation. Our findings need to be corrected and strengthened with the accumulation of detailed and accurate geomorphic and hydrological data with the help of modern technology in the future. Further understanding of the evolutionary path in a more quantic manner will support land practitioners in managing this ever-changing river system. Acknowledgments We thank the late Dr. Noel Trustrum, who provided us with valuable opportunities to work in the East Coast of North Island over several years, and consistent friendship. We also greatly appreciate the helpful and insightful comments provided by Ian Fuller and two anonymous reviewers, which substantially improved the first manuscript. References Bagnold, R.A., 1966. An approach to the sediment transport problem from general physics. US Geol. Surv. Prof. Pap. 422–I, I1–I37. https://doi.org/10.1017/ S0016756800049074. Beschta, R.L., 1983. Long-term changes in channel widths of the Kowai River, Torlesse Range, New Zealand. J. Hydrol. 22, 112–122. Columbus, J., Sirguey, P., Tenzer, R., 2011. A free fully assessed 15 metre digital

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