RESEARCH ARTICLE Downstream‐Propagating Channel Responses 10.1029/2018JF004734 to Decadal‐Scale Climate Variability Key Points: • Geomorphic analysis of stream gage in a Glaciated River Basin data in a glaciated river basin Scott W. Anderson1 and Chris P. Konrad1 indicates a downstream‐propagating vertical channel response to climate 1Washington Water Science Center, U.S. Geology Survey, Tacoma, WA, USA variability • The climate‐driven bed elevation response propagated downstream at 1 to 4 km per year; propagation rate Abstract Regional climate is an important control on the rate of coarse sediment mobilization and scaled closely with channel slope transport in alpine river systems. Changes in climate are then expected to cause a cascade of geomorphic • Comparisons with previously responses, including adjustments in downstream channel morphology. However, the mechanics and collected records suggest a regionally coherent relation sensitivity of channel response to short‐term climate variability remain poorly documented. In the Nooksack between climate variability and River, which drains a glaciated stratovolcano in Washington State, bed elevation changes were inferred from coarse sediment yield in shifting stage‐discharge relations at seven U.S Geological Survey stream gages. Decadal‐scale elevation sediment‐rich, glaciated basins trends can be explained as a downstream‐propagating channel response to regional climate variability, Supporting Information: where periods of persistent warm, dry (cool and wet) conditions corresponded to periods of aggradation • Supporting Information S1 (incision). The channel elevation response propagated downstream at a rate of 1 to 4 km per year; propagation rate scaled closely with channel slope. Historical trends in glacier extent and flood intensity both show some potential to explain climate‐sediment linkages, though assessing causation is complicated Correspondence to: by the shared climate signal in both records. Results show the influence of the Pacific Decadal Oscillation, S. W. Anderson, [email protected] with relatively high coarse sediment yields prior to 1950 and since 1980, and notably lower sediment yields from 1950 to 1980. Measured sediment yields from nearby glaciated basins corroborate this history, suggesting a regional consistency to these climate‐sediment linkages. These results document consistent Citation: Anderson, S. W., & Konrad, C. P. relations between climate, sediment supply, and downstream channel response at the basin scale, with (2019). Downstream‐propagating channel responses propagating downstream over periods of decades with little apparent attenuation. channel responses to decadal‐scale climate variability in a glaciated river Plain Language Summary The shape and form of a gravel‐bedded river is a function of the basin. Journal of Geophysical Research: amount of water and sediment supplied from upstream. Climate‐driven changes in the amount of Earth Surface, 124. https://doi.org/ 10.1029/2018JF004734 sediment supplied from higher‐elevation source areas may then cause changes in downstream channel form, resulting in changes in habitat suitability or flood conveyance. However, the linkages between climate, Received 26 APR 2018 sediment production, and downstream channel response are complicated and often difficult to monitor, Accepted 26 JAN 2019 leaving it unclear if or how rivers might respond to climate variability over societally important timescales of Accepted article online 5 FEB 2019 years to decades. In the glaciated Nooksack River in Washington State, we observe that changes in climate over the past century have resulted in distinct and consistent changes in channel bed elevation. Those changes appear first in the headwaters and then propagate downstream over a period of decades. Channel change in the upper river is then a response to climate about 20 years prior, while channel change in the lower river is a response to climate about 70 years ago. These results provide evidence that, at least in certain settings, short‐term climate signals influence downstream river systems and help define the timing and magnitude of those adjustments.

1. Introduction Alluvial rivers are dynamic systems that adjust their channel geometry, slope, planform, or roughness in response to changes in hydrology and/or sediment supply. Understanding how a given river system has responded, or will respond, to a given perturbation remains a central question in fluvial geomorphology, with application to modern civil engineering, river restoration work, and interpretation of historical or stratigraphic records. Of the many potential drivers of channel change, the influence of decadal‐scale climate has become a topic of increasing interest, particularly in alpine watersheds (Costa et al., 2017; East et al., 2017; Huggel et al., 2012; Published 2019. This article is a US Government work Lane et al., 2017; Leggat et al., 2015; Micheletti et al., 2015; Micheletti & Lane, 2016). Much of this research and is in the public domain in the USA. has been motivated by the potential for recent or forecasted changes in temperature, precipitation, glacier

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extent, or snowpack persistence to increase sediment yields, which may in turn cause downstream channel adjustments. Conceptually, this would represent a paraglacial sediment response (Ballantyne, 2002; Church & Ryder, 1972), in which there is a transient increase in sediment yields as glacier retreat exposes unstable or metastable unconsolidated material, coupled with the possibility that such a paraglacial response may be accelerated by a concurrent increase in the stream power available to move that sediment. Increases in stream power may occur as a result of increased glacier‐melt runoff, increased storm intensity, or a greater likelihood of precipitation falling as rain instead of snow. Paraglacial sediment responses were originally defined in the context of Holocene reworking of glacial sedi- ments from the last glacial maximum (Church & Ryder, 1972), and observations of watershed‐scale paragla- cial responses have typically involved Holocene‐timescale landscape response (e.g., Church & Slaymaker, 1989; Hinderer, 2001; Hinderer et al., 2013). Over shorter timescales (10–100 years), examples of a rapid landscape responses to changing climate and glacier extent have often involved adjustments in individual landforms (Curry et al., 2006; Welch, 1970). Decadal‐ to centennial‐scale relations between climate, glacier extent, and watershed‐scale sediment yield have often been complex, reflecting the complex interplay of sediment supply, transport capacity, and basin connectivity (Geilhausen et al., 2013; Hicks et al., 1990; Leonard, 1997; Menounos & Clague, 2008; Nussbaumer et al., 2011; Ohlendorf et al., 1997). Most studies comparing sediment yields to changes in climate or glacier extent have focused on the finer fraction of the sediment load transported in suspension, either through direct suspended sediment monitor- ing (Costa et al., 2017) or fine sediment accumulations in lakes (e.g., Leonard, 1997; Menounos & Clague, 2008). Fewer studies have assessed variations in the delivery and transport of coarser bed material, which is substantially harder to quantify but central to understanding downstream channel responses. Several recent studies that have managed to quantify coarse sediment yields in small to midsized watersheds in the Alps have documented an increase in coarse sediment yields since about 1980, concurrent with an increase in temperature and associated glacier recession (Lane et al., 2017; Micheletti & Lane, 2016). However, sediment yields did not vary consistently with empirically predicted transport capacity. Potential explanations for complex relations between sediment yields and transport capacity proposed by Micheletti and Lane (2016) and Lane et al. (2017) included a rapid winnowing of fines from unsorted glacial till, such that the critical shear required to mobilize material increased rapidly after initial exposure, or a lack of connectivity between various hillslope sources of glacial sediment and trunk outlet streams. These discus- sions of sediment yields from alpine watersheds are similar to more general discussions regarding whether climate signals are likely to be transmitted to depositional settings, given the nonlinear and complex internal dynamics of sediment delivery, transport, and storage (Van De Wiel & Coulthard, 2010; Jerolmack & Paola, 2010; Simpson & Castelltort, 2012; Romans et al., 2016). If and how sediment supply from alpine watersheds may respond to short‐term (decadal) changes in climate then remains difficult to predict. Presuming some signal of climate‐driven changes in sediment supply existed, the downstream channel response to that signal would be mediated by the routing of bed material and reach‐scale interactions of channel morphology, sediment transport, and stream flow (e.g., Gran & Czuba, 2017; Lisle, 2007). Much of the literature on that topic has focused on relatively punctuated increase in supply that results in distinct zones of sediment accumulation or increased sediment flux, variously referred to as sediment waves (e.g., Gilbert, 1917; Lisle et al., 2001), bed waves (James, 2006), sediment slugs (Nicholas et al., 1995), or sediment pulses (Cui & Parker, 2005), that tend to propagate and evolve downstream. Sediment inputs associated with these features have often been relatively discrete in both time and space, with well‐defined initial bed eleva- tion disturbances; examples include sediment inputs related to channel‐impinging landslides and debris flows or dam removals (e.g., Cui & Wilcox, 2008; Sutherland et al., 2002). Downstream‐propagating channel adjustments have also been documented in response to more spatially diffuse, basin‐scale processes that may increase sediment supply, such as intensive land use (typically logging or mining), large individual floods, or the combination of the two (Gilbert, 1917; Madej & Ozaki, 1996; Nelson & Dubé, 2016). Although wave‐like features have often been defined primarily in terms of bed elevation changes and cycles of aggradation and incision, there is a broad appreciation that channel responses may involve adjustments in channel planform, bed roughness, and grain size or sorting and that the relative importance of those various adjustments may vary between reaches with different configurations or valley confinement (Ferguson et al., 2015; Gaeuman et al., 2017; Lisle, 2007; Nelson et al., 2015). Though it is unclear if variations in climate may result in distinct, individually identifiable wave features, this literature provides a framework for discussing and

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a b

Figure 1. Example images of glaciated stratovolcanoes and downstream rivers in western Washington. (a) Photo of the sediment‐rich White River, a principal tributary of the Puyallup River, draining the North Flank of Mount Rainier. Photo is shown as representative of the topography and river morphology around the stratovolcanoes in the region; similar photos of the Nooksack River draining Mount Baker were not readily available. Photo credit: Elisa Johnson. (b) Recently deglaciated terrain down valley of the Deming Glacier on Mount Baker, which feeds into the Middle Fork Nooksack River. Exposed slopes of unconsolidated material in the foreground are about 250 m high with an average cross‐valley slope of about 25–30°. Photo credit: John Scurlock.

understanding how changes in upstream sediment supply may manifest and propagate in downstream channels.

1.1. Motivation The goal of this study was to assess whether historical changes in river bed elevations in a watershed drain- ing glaciated, sediment‐rich terrain on a stratovolcano in western Washington State, USA, could be related to decadal‐scale climate variations. Stratovolcanoes in the Cascades are steep, composed of friable volcanic rock, heavily glaciated, and experience high orographic precipitation, all of which make them a significant source of coarse sediment to down‐valley river systems (Czuba et al., 2010; Czuba, Magirl, et al., 2012). Like glaciers around the world (Marzeion et al., 2014), glaciers in the Cascade Range of Washington State have generally been retreating since reaching maxima in the eighteenth and nineteenth centuries (Burbank, 1981; Dick, 2013; Harper, 1993; Nylen, 2004). The potential combination of increased exposure of unconso- lidated sediments and more intense precipitation that is more likely to fall as rain (Salathé et al., 2014) has raised regional concerns that increased coarse sediment yields in glaciated headwaters might result in chan- nel aggradation, loss of conveyance, and increased flood risk in downstream reaches (Czuba et al., 2010).

1.2. Study Area This study focused on the 2,035 km2 Nooksack River, a basin in northwestern Washington State draining a prominent stratovolcano (Mount Baker, elevation 3,286 m) of the Cascade Range (Figures 1 and 2). The basin drains steep, mountainous terrain of the North Cascades in its headwaters and then flows through broad, glacially carved valleys in the lower‐elevation Puget Lowlands before exiting into Puget Sound. Most of the glaciated terrain in the basin is located on the flanks of Mount Baker and, to a lesser degree, Mount Shuksan, a 2,783‐m nonvolcanic peak to the east of Mount Baker. The main stem Nooksack is formed by the combined inputs from the North, Middle, and South Forks. The North Fork is typically considered the main fork, while the other two forks are considered major tributaries. Both the North Fork and Middle Fork Nooksack receive sediment from proglacial basins on Mount Baker and, in the North Fork, from Mount Shuksan. The South Fork has headwaters in lower‐elevation, ungla- ciated terrain. The North and Middle Fork Nooksack rivers are generally wide, dynamic anabranching or braided systems. In contrast, the South Fork has a predominately single‐threaded, alternating‐bar planform. The upper main stem river, downstream of the Middle and South Fork confluences, remains broad and active until 35‐m upstream from the mouth, where it becomes single threaded and confined between flood protection structures (Figures 2b–2d). The entire basin is unregulated but has experienced historical land use changes and development, including logging, clearing of in‐channel woody debris, and the construction of flood and bank protection structures.

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Figure 2. Site map of the Nooksack River. (a) The Nooksack Basin and gage locations. Gages are labeled as either North Fork (NF) or main stem (MS) gages and numbered sequentially downstream from one to seven. Inset shows the Nooksack basin, in red, in relation to Washington State, as well as the extent of National Oceanic and Atmospheric Administration's Climate Division 5. b–d) Aerial imagery over the Nooksack River, showing lower anthropogenically confined reaches near MS‐7 (b); the rapid transition from an unconfined to confined river system near MS‐5 (c); and the broad, braided planform representative of the North Fork, shown at its confluence with the Middle Fork (D). All imagery is from the 2013 National Agricultural Imagery Program.

2. Methods 2.1. Assessing Channel Change Changes in river bed elevation at U.S. Geological Survey streamflow gages were inferred from changes in stage‐discharge relations at those gage sites (Juracek & Fitzpatrick, 2009, Slater et al., 2015; Smelser & Schmidt, 1998; Table 1). Changes in stage‐discharge were quantified by comparing stage‐discharge informa- tion from all historical stream flow measurements against a baseline stage‐discharge relation, defined here using the most recent (ca. 2017 for active gages) stage‐discharge rating curve for a given gage (Figure 3). For each measurement, a stage residual was calculated as the difference between the measured stage and the stage expected based on the baseline rating curve, similar to methods used in James (1991). A positive stage residual indicates that at the time of the measurement, the stage associated with the measured discharge was higher than it would be at present, and a negative stage residual indicates that it was lower. Changes in stage‐discharge relations can result from changes in river bed elevation, cross‐section geometry, or channel roughness. For each gage, systematic changes in channel roughness or width were assessed by fitting a locally weighted scatterplot smoothing (Cleveland, 1979) curve to the relation between measured

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Table 1 USGS Gages Used in This Analysis Text River Drainage area, Period Mean annual 2‐year flood, Bankfull USGS gage ID USGS gage name label kilometer in km2 of record flow, in m3/s in m3/s Froude number

12205000 North Fork Nooksack near Glacier NF‐1 92.6 272 1938–present 23 171 0.64 12207200 North Fork Nookack near Deming NF‐2 64.1 731 1964–1975 47 275 0.64 12210500 Nooksack River at Deming MS‐3 53.5 1,514 1935–2005 95 722 0.48 12210700 Nooksack River at North Cedarville MS‐4 45.1 1,527 2003–present 107 — 0.47 12211200 Nooksack River at Everson MS‐5 35.3 1,628 2008–present 112 — 0.47 12211500 Nooksack River near Lynden MS‐6 22.5 1,680 1944–1967 105 722 0.31 12213100 Nooksack River near Ferndale MS‐7 7.6 2,037 1966–present 110 722 0.25 Note. USGS = U.S. Geological Survey.

mean water depth and discharge for the measurements. For each measurement, the residual above or below that locally weighted scatterplot smoothing curve was then calculated, analogous to how stage residuals were calculated. Since the mean depth is independent of the absolute channel elevation, the depth‐ discharge residuals isolate the component of the stage‐discharge residual that is attributable to changes in width or roughness. We found no systematic trends in depth‐discharge relations at any of the sites assessed here, indicating that trends in the stage‐discharge relations observed in the Nooksack can be attributed primarily to changes in the mean elevation of the wetted river bed. Changes in the shape of the channel cross section may cause the slope of a rating curve to change over time, resulting in divergent trends in stage residuals made at different measurement discharges. For example, widening of the low‐flow channel would create negative stage residuals at low flows but may have little impact on the stage at high flows. In contrast, if the slope of the stage‐discharge relation has remained stable over time, the stage‐residual should be relatively independent of the measurement discharge. At all gages used in this study, short‐term variability in stage residuals was found to be largely independent of the

Figure 3. Example stage‐discharge residual analysis, using records at NF‐1 (North Fork Nooksack near glacier, U.S Geological Survey 12205000). (a) Plot showing 2016 rating curve, along with four measurements selected to span the period of record at the NF‐1 gage. The date of each measurement is noted. The vertical distance between each measure- ment and the rating curve is the stage residual. (b) Stage residuals for four selected measurements plotted over time. (c) Same as (b), but using all measurements made at or below 150% of the mean‐annual flow. (d) Stage‐discharge residuals over the past 20 years, showing periods of gradual adjustment and two distinct fill and scour events. Points are colored according to the discharge at time of measurement, demonstrating the absence of discharge‐dependent variability and indicating stability in the slope of the rating curve over time. NF = North Fork.

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measurement discharge for measurements made at or below 150% of the local mean annual discharge (Figure 3d), indicating a consistent rating‐curve slope over this range of low‐to‐moderate discharges. Stage residuals were often erratic for the small fraction of measurements made at higher flows, indicating poten- tially confounding influences of channel roughness (e.g., vegetation), lateral sediment deposition, and hys- teresis in stage‐discharge relations for floods, as well as greater measurement uncertainty. Long‐term records of vertical channel change were then based on stage‐residuals from all measurements made below 150% of the local mean annual flow. This resulted in about five to seven data points per year at a given gage. The resulting records of stage residuals represent channel adjustment at the hydraulic control for the gage site during low flows, which is typically the first riffle downstream of the gage pool. Given the lack of observed changes in depth‐discharge relations, the stage‐discharge residuals primarily represent changes in the mean elevation of the downstream riffle cross section.

2.2. Characterizing Regional Climate Annual variation in regional climate was characterized using a climate index modeled after the Pacific Northwest Index (PNI) of Ebbesmeyer and Strickland (1995). This sort of composite climate index was selected because it captures the decadal phases of climate that have been shown to influence a wide range of glacial, hydrologic, and ecologic processes in the region (Ebbesmeyer & Strickland, 1995; O'Neal, 2005; Stoelinga et al., 2010). The PNI was originally defined by Ebbesmeyer and Strickland in terms of three long‐term hydro‐meteorological records in western Washington; the annual temperature anomaly at Olga, WA, the annual precipitation anomaly at Cedar Lake, WA, and the 15 March snowpack anomaly at Paradise, WA. All records were first recast as annual anomalies by subtracting the period of record mean and then dividing by the standard deviation, and the index was calculated as the mean of the temperature anomaly, the negative precipitation anomaly, and the negative snowpack anomaly. The index was then most positive in warm years with low precipitation and snowpack and most negative in cool years with high pre- cipitation and a large snowpack. The combination of warm and dry versus cool and wet reflects the typical modes of climate variability in the region. In this study, a similar index was calculated using only temperature and precipitation anomalies based on spatially averaged climate reconstructions (Figure 4). Annual temperature and precipitation from 1895 to 2016 were obtained from the National Oceanic and Atmospheric Administration's nClimDiv data set, a gridded (5‐km) product based on climatologically informed interpolation of historical station records (Vose et al., 2014). Annual values were spatially averaged over NOAA's Washington State climate division 5, cover- ing the western slope of the Cascade Range (Figure 2), and standardized by subtracting the period of record mean and dividing by the period of record standard deviation. The climate index was calculated as the mean of the temperature anomaly and the negative precipitation anomaly for a given year. The index is then most positive when the region was both warm and dry and most negative when the region was both cool and wet. Note that “wet” here describes the total precipitation over an entire year, including precipitation falling as snow, and does not directly correspond to high‐intensity precipitation events and flooding. As will be shown, periods of higher total precipitation have historically been associated with less intense flooding and vice versa. The climate index calculated here is strongly correlated with the three‐station PNI of Ebbersmeyer and Strickland (Pearson's r of 0.83 for period of record from 1985 to 2016, increasing to 0.88 for the 5‐year run- ning mean of both metrics), indicating that the trends in regional climate over the past century are relatively homogeneous and insensitive to the spatial extent of hydro‐meteorological data. The climate index shows several decadal‐long periods of either cooler and wetter or warmer and drier weather. Those periods correspond to distinct phases of the Pacific decadal oscillation (PDO), which is defined in terms of sea‐surface conditions in the North Pacific and has a strong influence of the storm track for the Pacific Northwest (Figure 4; Mantua et al., 1997; Mantua & Hare, 2002). The regional climate index is significantly correlated with the PDO index (r = 0.53, increasing to 0.64 for the 5‐year running mean), where cool and wet years are associated with negative PDO indices.

2.3. Characterizing Potential Physical Mechanisms Changes in mean annual temperature and total precipitation are unlikely to be direct controls on sediment production and transport in the Nooksack River. Instead, the climate index calculated here is hypothesized to act as a proxy for a variety of processes that do control sediment processes and are either likely to respond

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Figure 4. Regional climate data from the National Oceanic and Atmospheric Administration's nClimDiv data set, aver- aged over Washington State climate division 5 (western Cascades). (a) Mean annual temperature anomalies. Temperature and precipitation anomalies are shown after centering values by the mean of the period of record, but before division by the standard deviation, in order to retain physical units. (b) Total annual precipitation anomalies. (c) Regional climate index, calculated as the mean of the temperature anomaly and negative precipitation anomaly. (d) Mean annual value of the Pacific decadal oscillation (PDO) index; data are from the Pacific Decadal Oscillation website (http://research.jisao.washington.edu/pdo). Smoothing in all panels is a 7‐year Gaussian‐kernel moving average.

to changes in annual temperature and precipitation (such as glacier extent or snowpack persistence) or correlate with them (such as the timing, intensity, and/or frequency of large precipitation events). Changes in glacier extent and changes in headwater storm intensity were assessed here as two plausible processes that may link climate, defined as per our climate index, and sediment yields. Records of total glacier area on Mount Baker were obtained from Harper (1993) and Dick (2013). Changes in headwater storm intensity were quantified based on discharge at the upstream‐most NF‐1 gage, with a per- iod of record from 1938 to present. Storm intensity was characterized in two ways; first, as the annual instan- taneous peak discharge, indicating the largest flood in a given year, and second, as the total streamflow volume passing the gage at discharges above 70 m3/s (2% exceedance interval) during the fall months. This second metric integrates information about both the intensity and frequency of storms during the fall

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season, when precipitation is more likely to fall on snow‐free hillslopes and produce debris flows in progla- cial basins (Anderson & Pitlick, 2014; Copeland, 2009; Legg et al., 2014; Walder & Driedger, 1994).

2.4. Cross Correlation of Records Initial visual inspection of the climate and gage records suggest lagged relation between climate and gage records, as well as between the gage records themselves. These lagged relations were formally assessed through cross‐correlation analysis, where one time series is progressively shifted relative to another in order to find the lag or lead time that provides the best correlation between the records. In order to define the arrangement of records that provided the best overall fit, a metric of overall agreement was defined as the mean cross‐correlation for all overlapping record pairs. Arrangements of records were described in terms of lag times for each gage record relative to the fixed climate record; those lag times then define lags between the gage records themselves. For example, if the NF‐1 and NF‐2 records were lagged relative to the climate record by 20 and 25 years, respectively, the overall coherence of this three‐record sub- set would be quantified as the mean of the 20‐year lagged correlation between climate and NF‐1, the 25‐year lagged correlation between climate and NF‐2, and the 5‐year lagged correlation between NF‐1 and NF‐2. The arrangement that provided the highest mean correlation across all overlapping record pairs would then be considered the best overall arrangement, even if the cross‐correlation between any given pair of records might not be a true local peak. Details of these methods are presented in the supplemental materials.

3. Results 3.1. Vertical Channel Change in the Nooksack Basin Changes in stage‐discharge relations at all seven gages on the North Fork and main stem Nooksack River indicate dynamic variability in bed elevation over a range of 0.5–1.0 m (Figure 5a). All sites exhibit persistent trends of incision or aggradation lasting years to several decades, often with relatively consistent rates of long‐term change. Rates of adjustment associated with those decadal‐scale trends were typically 0.2–0.4 m per decade. Over periods of less than several years, coherent changes most often occurred as pulses of fill and subsequent scour. These pulses were most readily apparent in the upstream‐most NF‐1 and MS‐3 gages but largely absent at the MS‐6 and MS‐7 sites. Discharge had little predictive power in terms of the direction or magnitude of channel adjustments (Figure 6). Although most large abrupt changes are associated with large floods, all three long‐term sites show that floods of a given magnitude do not have consistent impacts and may result in incision, aggrada- tion, or little change.

3.2. Relation to Regional Climate Analysis of pair‐wise cross‐correlation results indicated that five of the seven gage records (NF‐1, NF‐2, MS‐3, MS‐6, and MS‐7) could be arranged such that all gage records had statistically significant lagged correlations with the climate record, all overlapping gage‐record pairs were significantly correlated, and the lag between climate and channel response progressively increases with downstream distance (Figure 5 and Tables 2 and 3). The two remaining records, MS‐4andMS‐5, were considered too short and indistinct to situate within this broader arrangement. The exact arrangement selected here was based on maximizing the overall mean cross‐ correlation across all overlapping record pairs. A comparison of the 505 possible arrangements of these five records formed by allowing each gage record to independently lag climate over a 50‐year window verified that the arrangement selected here provided the highest overall coherence, in terms of the mean cross‐correlation of all record pairs, over that sample space. Details of these results are presented in the supporting information. Physically, the arrangement of records identified here implies that variations in climate have an impact on coarse sediment delivery from the headwaters upstream of the NF‐1 gage, which then result in regular channel responses that propagate downstream. The lag between climate forcing and channel response ranged from 20 years at the NF‐1 site, in the upper North Fork, to nearly 70 years at the MS‐7 gage, near the river's mouth (Figure 5b). The climate signal then took about 50 years to travel the 90 km between those bounding gages. Periods of relatively warm and/or dry climate corresponded with subsequent periods of aggradation, while per- iods of cool and/or wet climate corresponded with subsequent periods of incision. Variations in climate were able to explain slightly more than 40% of the variance in channel elevations at the five gage sites (Table 2).

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Figure 5. Vertical channel change inferred from stage‐discharge residuals at U.S. Geological Survey stream gages in the NF and MS Nooksack Rivers. (a) Stage residuals for all gages. Gages have been plotted in upstream to downstream order; exact vertical position is arbitrary. (b) Lag relative to the climate record that produces the best overall fit of the five arranged records. (c) Arrangement of the five gage records with climate, based on best fit lags. The arrangement of the two gages not initially fit was estimated using the observed relation between slope and signal celerity (Figure 7). Dashed lines show the magnitude of the lag (in years) applied to each record; finer dashes indicate lags estimated using slope‐ celerity relation. NF = North Fork; MS = main stem.

The magnitude of the channel response to a given climate forcing did not differ appreciably among the gages (Figure 5c). As a result, records of channel change in the lower river can be interpreted as an estimate of channel change at upstream gage sites at some point in the past, while records of channel change in the upper river provide a forecast of expected channel change in the lower river in the future. The arrangement of all five gage records results in a 100‐year composite record of the “characteristic” pattern of channel change that we would predict has occurred, or will occur, at each gage.

3.3. Celerity of the Bed Response The lag time between different gages provides a measure of the speed, or celerity, of the climate signal pro- pagating downstream (Table 3). Signal celerity ranged from about 1 to 4 km per year. This is similar to pre- vious estimates of migration rates for channel disturbances or adjustments, which have typically ranged from several hundred meters to several kilometers per year (Beschta, 1983; Goff & Ashmore, 1994; Griffiths, 1993; Knighton, 1989; Madej & Ozaki, 1996; Nelson & Dubé, 2016; Roberts & Church, 1986).

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Figure 6. Change in stage‐residual between sequential measurements compared with maximum daily discharge over that interval for the long‐term gage records at NF‐1 (a), MS‐3 (b), and MS‐7 (c) gage sites. Note log scale on the x‐axis. NF = North Fork; MS = main stem.

Signal celerity over a given reach scaled closely with the mean slope of that reach, such that the signal propagated fastest in the headwaters and progressively slowed as it moved downstream (Figure 7). These results are similar to observations that the celerity of a bed wave moving down Redwood Creek, northern California, decreased as it moved downstream to reaches with lower unit stream power (Madej & Ozaki, 1996). Quantifying the relation between slope and signal celerity was complicated by the nature of averaging rates of travel. Presuming a perfect linear relation between slope and celerity existed, a comparison between average celerity and average slope calculated over long reaches over which the slope changed would not exactly recover that linear relation. This is because an average rate of travel is the harmonic mean (inverse of the mean of the inverses) of the rates of travel over subreaches, while the mean slope is effectively the arithmetic mean of those subreach slopes. Calculating mean channel slope as the harmonic mean, based on 1‐km subreaches, provided more directly comparable results and indicates that signal celerity is well described as either a linear or power law function of channel slope. A linear fit nominally provides a higher r squared value than the power law fit (0.98 vs. 0.94), though the power law (celerity = 46.63 * slope0.51) has the physically attractive property of asymptotically approaching zero as channel slope goes to zero and is shown in Figure 7. The consistent relation between channel slope and signal celerity means lag times for the two short periods of record (MS‐4 and MS‐5), which were not initially fit in the arrangement, can be estimated. Estimated arrange- ments of those two records show reasonable agreement with the rest of the records, particularly in the case of the MS‐5 record (Figure 5c). The MS‐4 record shows good overall agreement over the first 7 years of record, when all overlapping records were aggrading, but has experienced incision over the latter 7 years that has been more persistent than would be expected based on overlap with the NF‐1 and climate records. While the short periods of record necessarily provide equivocal results, the fit of these two records, based purely on slope‐ predicted signal propagation rates, leaves open the possibility that trends at these two sites may also be interpreted as part of the overall climate Table 2 response, albeit with some locally divergent trends at the MS‐4 site. Best Fit Lags Between the Climate Record and a Given Gage Record River Lag with R2 in best fit a Gage ID kilometer climate, years arrangement 3.4. Relation to Physical Mechanisms NF‐1 92.6 21 0.41 (p << 0.001) The observed correlations between climate and channel response do not ‐ p NF 2 64.1 28 0.44 ( = 0.020) provide any indication of what physical process or processes link MS‐3 53.5 31 0.45 (p << 0.001) MS‐4 45.1 —— variations in climate to variations in sediment yield; it is unlikely that sedi- MS‐5 35.3 —— ment dynamics would be directly sensitive to changes in mean temperature MS‐6 22.5 55 0.60 (p << 0.001) or total precipitation, and the climate index is presumed to be acting as a MS‐7 7.6 70 0.41 (p << 0.001) proxy for other relevant processes. Changes in glacier extent and headwater aBest fit arrangement is defined as the one that maximizes the mean storm and/or flood intensity were assessed here as two plausible climate‐ cross‐correlation between all record pairs; see the supporting information. related physical drivers.

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Table 3 Lags Between Sequential Gages, With Calculated Signal Celerity Estimated R2 in best fit Channel distance Mean channel Harmonic mean Signal celerity, Start site End site lag, yearsa arrangementb between sites, kilometers slopecc channel slopec km/year

NF‐1NF‐2 7 0.43 (p << 0.001) 29.9 0.0091 0.0080 4.3 NF‐1MS‐3 10 0.61 (p = 0.020) 39.2 0.0078 0.0067 3.9 NF‐2MS‐3 3 0.20 (p << 0.001) 9.3 0.0038 0.0044 3.1 NF‐1MS‐634 — 70.0 0.0054 0.0024 2.1 NF‐2MS‐627 — 40.1 0.0024 0.0015 1.5 MS‐3MS‐624 — 30.8 0.0016 0.0013 1.3 NF‐1MS‐7 49 0.02 (p = 0.02) 85.1 0.0043 0.0012 1.7 NF‐2MS‐742 — 55.2 0.0018 0.0009 1.3 MS‐3MS‐7 39 0.19 (p << 0.001) 45.8 0.0013 0.0007 1.2 MS‐6MS‐7 15 0.62 (p << 0.001) 15.1 0.0004 0.0004 1.0 aLags between gage pairs estimated based on best fit arrangement of all records (Table 2). bR2 values only calculated for gage pairs with at least 5 years of overlap in best fit arrangement. cMean or harmonic mean slopes were calculated over the reach bounded by a given gage pair.

Total glacier area on Mount Baker has generally been decreasing since reaching a Little Ice Age maxima sometime in the nineteenth century (Dick, 2013; Harper, 1993), with a period of particularly rapid retreat from about 1930 to 1945 followed by readvances from 1945 to 1980 (Figure 8). A similar sequence of glacial retreat and advance has occurred on the other regional stratovolcanoes (Jackson & Fountain, 2007; Nylen, 2004; Sitts et al., 2010) as well as in southwest British Columbia (Menounos, 2006; Koch et al., 2009). Since around 1930, these trends in glacier extent are well explained by variations in regional climate, and, consequently, changes in glacier extent also correlate well with records of channel change. However, prior to 1930, regional climate was generally cool and wet (relative to the 1895–2016 mean), while glacier area on Mount Baker continued to decrease. Similar divergent trends were observed on Mount Rainier during this same period (Burbank, 1981) and indicate that glaciers in the early twentieth century were still responding to a regional increase in temperature in the late nineteenth century (Burbank, 1982; Graumlich & Brubaker, 1986). Records of channel change corresponding to this time show an incisional signal, tracking the climate record and not the counter‐veiling glacier trend (Figures 5c and 8). This implies that glacier extent has not been the sole control on headwater sediment delivery and leaves open the possibility that correla- tions between glacier extent and channel change since 1930 may be shared responses to climate but not physically linked. Both metrics of headwater storm intensity indicate an increase in that intensity in the late‐twentieth century (Figures 8c and 8d). Annual instan- taneous peak discharges experienced a step increase around 1978, coinci- dent with the PDO shift (Figure 4d). The early 2000s experienced several large floods, though it is unclear if these more recent events truly repre- sent a shift in the underlying flood frequency distribution. Starting in 1990, there has been a significant and relatively aburpt increase in the upper range of fall storm intensity; over the 50 years prior to 1990, no fall storm season ever passed more than 200,000 m3 of stream flow at flows above 70 m3/s. Since 1990, that threshold has been passed in 7 of the past 27 years, including 3 years that passed nearly 600,000 m3. Years in which fall discharges never exceeded 70 m3/s have also become less frequent fl Figure 7. Signal celerity as a function of harmonic mean channel slope. since the late 1970s. Note that ood intensity has then generally been Signal celerity was estimated based on the lag time and channel distance higher during periods of nominally warm and/or “dry,” in terms of total between each of the five gages aligned with the climate record. Harmonic precipitation, climatic periods. mean (the inverse of the mean of the inverses) slopes were calculated based on regular 1‐km subreaches of the channel. Data from the four independent For both metrics, the timing and direction of these shifts could plausibly reaches between the five gages are indicated. be related with the observed transition from incision to aggradation at

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Figure 8. Historical trends in glacier extent and flood hydrology in the Nooksack and their relation to regional climate. (a) Climate index, shown as the annual value (gray bars) and the cumulative departure from the mean (black line). (b) Changes in total glacier area on Mount Baker and cumulative terminus retreat of the Nisqually Glacier on Mount Rainier. Nisqually terminus retreat data are presented to document that progressive retreat was likely occurring on Mount Baker in the decades prior to 1930. Note that the y‐axis for total glacier area has been reversed; increasing values indicate loss of glacier area. (c) Annual instantaneous peak discharges for the NF‐1 (U.S Geological Survey 12205000) gage. (d) Fall flood activity, calculated as the total flow volume for fall (October–December) days with daily mean discharge above 70 m3/s at the NF‐1 gage. Dashed lines in (c) and (d) are the cumulative departures from the mean for the respective flood metrics. NF = North Fork.

the NF‐1 gage and would indicate that increased headwater sediment delivery associated with larger storms outweighed the concurrent increase in transport capacity of large floods. However, the available records of flood hydrology do not show any obvious shifts that might explain the aggradation observed at the NF‐1 gage in the mid‐1960s or the subsequent turn toward sustained incision. These results indicate that both glacier extent and flood hydrology may plausibly be physical mechanisms linking climate and sediment dynamics, though neither record can independently explain all the features of the channel response. Moreover, the observation that both glacier extent and flood intensity experienced distinct shifts in the late 1970s, coincident with a shift in the PDO and regional climate, indicate that asses- sing causal physical links through correlation of time series may be complicated given that multiple poten- tially important physical processes are likely responding to the same broad climatic forcing. Understanding the physical links in this system will presumably require more direction observations of the rates and mechanisms of sediment production and transfer occurring in the headwaters.

4. Discussion 4.1. Climate as a Control on Channel Response In the Nooksack River, consistent lagged correlations between climate variability and channel response sug- gest a dynamic wherein climate variability drives changes in sediment supply in the headwaters upstream of

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the NF‐1 gage; those sediment supply variations then result in channel adjustments, manifesting at our gage sites primarily as elevation adjustments, that propagate downstream over periods of decades. These climate‐ driven bed elevation trends persisted over 90 km of downstream transport and were the dominant compo- nent of long‐term trends in at least five of the seven gage records. These results are significant both in that they indicate a consistent coarse sediment response to short‐term climate variability in the Nooksack Basin over the past century and document that those sediment supply signals propagated downstream over periods of decades, with a celerity closely related to channel slope. These results bridge studies documenting rela- tions between moderate‐term (<1,000 years) climate variability and sediment or channel dynamics based on static stratigraphy (i.e., Knox, 1984; Macklin et al., 1992; Menounos & Clague, 2008; Rumsby & Macklin, 1994) and studies focused on dynamic, transient channel responses to sediment supply changes that, given sufficient amplitude, would presumably be responsible for building such stratigraphic records (e.g., Gilbert, 1917; Madej & Ozaki, 1996). The fact that downstream channel responses show consistent relations to climate variability requires that (1) short‐term variations in climate result in consistent changes in upstream supply and that (2) the signal of those supply variations persist downstream without dissipating or becoming obscured by other interfer- ing signals. While the details of the physical mechanisms involved in generating and transmitting these sig- nals remain largely unknown, we suggest that several features of the Nooksack Basin may make it more likely for these conditions to be met. First, the likely significant sediment delivery from Mount Baker, both in terms of ongoing erosion of the edifice and re‐entrainment of debris flow and lahar deposits in upper val- leys, may provide a relatively localized supply zone that may produce in a single climate signal, instead of many interfering signals generated from spatially distributed subbasins. Second, many of the headwater basins are characterized by active glaciation, large volumes of unconsolidated, and unvegetated sediment and experience large fall and winter rainfall‐driven floods, owing to the proximity to coastal moisture and the orographic influence of the Cascade Range and the stratovolcanoes in particular. The combination of high sediment supply and large floods likely creates a situation where coarse sediment is moving down- stream regularly. Consequently, climate‐related variations in the relative sediment supply or the intensity and/or frequency of events that move that sediment would seem more likely to result in direct changes in sediment delivery to downstream reaches, instead of being confounded by supply limitations (e.g., time lags for debris flow hollows to recharge between events) or the rapid development of armored glacial or debris flow deposits that limit delivery responses to changing streamflow (Lane et al., 2017). In combina- tion, the presence of a significant, spatially localized source area in which short‐term changes in climate are likely to have a strong influence on rates of sediment export may plausibly make the Nooksack Basin particularly amenable to the generation and persistence of these climate signals. The specific history of cli- mate that has occurred in the region, with several distinct climatic periods separated by relatively abrupt shifts, may also have been important in generating a sufficiently distinct channel response that could be identified and tracked. Our interpretation focuses on alpine headwaters, and particularly those on Mount Baker, as natural long‐ term sources of coarse sediment to the Nooksack River basin. Although deforestation and associated road building have occurred in the basin and may have had some influence on coarse headwater sediment pro- duction (i.e., Madej & Ozaki, 1996), there are several reasons to think that influence is minor in the context of our results. First, sediment production from the stratovolcanoes is likely to be substantially larger (order of magnitude) than from forested catchments subject to land use (Czuba, Olsen, et al., 2012). This is consistent with planform characteristics in the Nooksack, where the North and Middle Forks that drain Mount Baker are often broad and braided, indicative of high coarse sediment loads, while the South Fork is generally nar- rower and has a meandering planform. Second, the fact that the sediment supply signal originated some- where in the basin above the NF‐1 gage, and was not substantially influenced by downstream sediment inputs, is difficult to reconcile with the spatial pattern of land use. Qualitative examinations of aerial imagery extending back to the 1940s indicate that logging has been focused in the lower‐elevation, more‐accessible areas of the South and Middle Fork Nooksack. Stand‐age inventories (USDA, 1998) indicate that only about 10% of the NF‐1 basin has ever been logged since 1850. Both the planform characteristics of the three major forks and the spatial progression of the climate signal are then consistent with sediment‐rich alpine water- sheds as the dominant source of coarse sediment, and the source of the supply‐change signal, but inconsis- tent with the spatial pattern of land use.

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Figure 9. Historical sediment yields and variability from glaciated terrain in the Pacific Northwest. (a) The regional cli- mate index, plotted both as annual values (gray bars) and as the cumulative departure from the mean (black line). (b) Total sediment yield variations from the south flank of Mount Rainier, based on repeat bathymetric surveys of Alder Lake made in 1956, 1985, and 2011 (Czuba, Olsen, et al., 2012). (c) Relative accumulations of fine sediment in Cheakamus Lake in southwest British Columbia (Menounos & Clague, 2008). Records are based on the average of multiple cores. Mean accumulation depths were first log transformed and then normalized using the log‐transformed mean and standard deviation from 1000 to 2000 CE, as originally presented in Menounos and Clague (2008). Cheakamus Lake is presented as broadly representative of trends observed in multiple lake core studies in the region.

4.2. Comparison to Regional Sediment Yields Although coarse sediment yields in the Nooksack have not been directly documented over the past century, the periods of higher or lower yield inferred from channel change in the Nooksack are corroborated by sev- eral independent records of sediment yield elsewhere in the region. The most direct comparison comes from repeat bathymetric surveys of Alder Lake. Alder Lake traps most of the sediment, and all of the bed material, delivered from the south flank of Mount Rainier, another regional stratovolcano located 200 km south of Mount Baker (Czuba, Olsen, et al., 2012). Additional records come from several varved lake cores collected in lakes draining glaciated terrain in southwest British Columbia, which provide annually resolved records of relative fine‐sediment yields over the past several centuries (Leonard, 1997; Menounos, 2006; Menounos & Clague, 2008; Heideman et al., 2015). Across these various records, there is a consistent decline in sediment yield in the middle of the century, and a subsequent increase in the 1970s or 1980s, tracking regional shifts in climate and matching transitions toward incision and aggradation, respectively, in the Nooksack (Figure 9). The long‐term lake core records indicate that fine‐sediment yields in the early part of the twentieth century were some of the highest over the past century (Menounos, 2006), while yields from 1950 to 1980 were some of the lowest (Menounos & Clague, 2008). A similar sequence of channel narrowing and widening was also identified in multiple rivers in the Olympics, located about 150 km southwest of the Nooksack Basin, and also related to climatic periods of higher or lower flood intensity (East et al., 2017). These regional observations then both corroborate the history of sediment yields inferred in the Nooksack based on bed elevation changes and indicate a regional coherence shared across these sediment‐rich, gla- ciated basins. The pacing of these historic variations largely track climate variability associated with the PDO; recognizing the influence of multidecadal natural climate variability on sediment delivery, and the potential for lagged downstream channel responses, may often then be central to disentangling geomorphic cause and effect in regional watersheds.

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4.3. Channel Responses as a Bed Wave Observations of downstream‐propagating patterns of vertical channel response in the Nooksack have clear analogs to the literature on sediment or bed waves, and particularly to literature documenting basin‐scale channel responses to spatially diffuse sediment inputs (Gilbert, 1917; Madej & Ozaki, 1996). However, whereas nearly all prior studies have discussed a single rise and fall of the channel bed associated with a sin- gle period of increased supply, the Nooksack has experienced multiple rises and falls, as well as periods of relatively stability, all of which have been replicated at progressively downstream gages. Although this sequence of responses could plausibly be described as a sequence of overlapping bed waves, we have elected to refer to it more generically as a signal of changing sediment supply. This decision was made largely because discussions of sediment or bed waves has become strongly associated with those individual, punc- tuated disturbances, typically discussed in reference to some initial equilibrium bed elevation and as a dis- tinct from “normal” background processes. This conceptual model of an equilibrium channel disturbed by a punctuated increase in supply is difficult to apply in the Nooksack. There is no basis for defining an equi- librium elevation at any of our gage sites, and it instead seems more reasonable to see our results as indica- tive of a constant state of disequilibrium. Further, it seems unlikely that variations in climate would result in any singular sediment supply event; instead, we judge it more likely that channels in the Nooksack are responding to variations in the average frequency or intensity of the landslides, debris flows, and large floods that collectively define the long‐term average sediment input. Regardless of whether observations in the Nooksack fit the formal definition of a bed wave or a sequence of bed waves, there is a clear commonality in that signals of upstream sediment supply changes are being pro- pagated downstream. Within that context, a persistent topic of interest has been whether such signals are likely to attenuate, or decrease in amplitude, moving downstream. This has practical importance for under- standing the likely magnitude of downstream impacts to a given upstream event and has resulted in a long‐ running discussion whether waves are likely to evolve through translation, implying no downstream attenuation, or through dispersion, which do attenuate as the move downstream (Lisle, 2007). In addition to a constant amplitude, translating waves also involve downstream migration of both the leading and trail- ing edge of the wave. In contrast, the trailing edge of a dispersive wave is either stable or, in some cases, may effectively migrate upstream as sediment is deposited in an upstream impoundment or zone of reduced gradient. In the Nooksack, we observe no apparent decrease in signal amplitude moving downstream. We also observe that incisional signals, analogous to the trailing edge in a single wave, moved downstream at the same rate as aggradational signals (if they did not, we would see significant distortions in the bed elevation trends as zones of aggradation and incision spread apart or began to combine). Patterns of channel change in the Nooksack then match typical definitions of a translation‐dominated wave. This is, to our knowledge, the first observation of a basin‐scale channel bed response that has not involved substantial attenuation moving downstream (Gilbert, 1917; James, 2006; Madej & Ozaki, 1996). Although we have a limited view of the mechanisms and longitudinal form of that channel response, the Nooksack response may differ from prior observations in part because of its relatively low amplitude. Sediment inputs related to hydraulic mining in the Sacramento Basin (James, 2006) and to logging and large floods in Redwood Creek, northern California, (Madej & Ozaki, 1996) resulted in aggradation of many meters that often completely buried valley floors, caused dramatic changes in channel planform, and placed large volumes of coarse sediment in floodplain or terrace storage. In contrast, channel elevations in the Nooksack have varied, at most, about 1 m, and the general character of the river has not obviously changed over time (Collins & Sheikh, 2004). Given the limited vertical adjustments and the high rates of lateral mobi- lity in much of the basin, it seems unlikely that large fractions of sediment stored during relatively aggraded periods would end up in isolated or perched position, reducing the possibility of long‐tailed recovery periods and increasing the likelihood that the system may be operating in a window where storage‐transport rela- tions are essentially linear and symmetric (Lisle & Church, 2002).

Average rates of elevation change related to climate variability have been on the order of about 1 to 5 cm per year and, in many cases, would be indistinguishable from short‐term variability over periods of less than about a decade. Given that the signal has propagated downstream at rates on the order of kilometers per − year, this implies an effective maximum aspect ratio of about 0.05/1,000 = 10 5, making it substantially

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more diffuse than any previously discussed wave feature (Figure 4 in Lisle et al., 2001). The simplest argu- ment for wave diffusion is that the increase in slope on the steeper leading face would tend to increase trans- port rates while the decrease in slope on the trailing would tend to decrease transport rates, naturally causing an initially coherent form to spread. One possible explanation for the lack of attenuation in the Nooksack is that the signal is already so diffuse that reach‐scale adjustments in slope related to that signal are too small to meaningfully influence its evolution. The lack of signal attenuation, with analogies to a translating wave, does not imply every reach down the length of the Nooksack may have experienced an identical sequence of vertical channel adjustments. Our observations of elevation change are located exclusively at gage sites, which are preferentially sited in reaches with strong lateral confinement. In the Nooksack, all five of the upstream‐most gages are located in short, single‐threaded reaches confined by bedrock or valley terraces and are distinct from the wide, unconfined, and often braided or anastomosing character of most of the upper river. We have no data to indi- cate how those unconfined reaches may have responded to climate‐related changes in sediment supply, and there is ample reason to suspect it may differ significantly from our gage reaches (Lisle, 2007; Nelson et al., 2015). However, the simple fact that we observe the signal at sequential gages implies that the intervening reaches must have experienced some suite of channel adjustments that allowed the upstream supply signal to continue to propagate downstream.

5. Conclusion Variations in regional climate in the Pacific Northwest appear to have resulted in regular variations in upstream coarse sediment supply in the Nooksack basin, resulting in a sequence of channel elevation responses that have propagated downstream over periods of decades. These results demonstrate that signals of short‐term climate variability have the potential to be transmitted down the length of a large basin, with the rate of downstream propagation controlled by channel slope. Regionally consistent records of higher or lower sediment yields over time both corroborate the climate‐sediment relations we infer in the Nooksack based on vertical channel change and suggest that those relations are shared across sediment‐rich basins in the region. However, the specific physical mechanisms that generate that initial supply signal remain unclear, as do the detailed mechanics of the downstream propagation of that signal. How that signal may Acknowledgments manifest in unconfined reaches is also unknown. The results presented here will hopefully provide motiva- The authors would like to acknowledge the support of Paula Harris and John tion and a framework for further exploration of these questions. Thompson with the Whatcom County Public Works for funding and encouragement. Brian Menounos kindly supplied data from Cheakamus References Lake. Reviews by Jim O'Connor, David Anderson, S. W., & Pitlick, J. (2014). Using repeat lidar to estimate sediment transport in a steep stream. Journal of Geophysical Research: Gaueman, Jon Czuba, and Tim Beechie Earth Surface, 119, 621–643. https://doi.org/10.1002/2013JF002933 all significantly contributed to Ballantyne, C. K. (2002). A general model of paraglacial landscape response. The Holocene, 12(3), 371–376. https://doi.org/10.1191/ improving the analysis and 0959683602hl553fa presentation of this work. All USGS Beschta, R. L. (1983). Long‐term changes in channel widths of the Kowai River, Torlesse Range, New Zealand. Journal of Hydrology (New data, including stream‐flow Zealand), 22(2), 112–122. measurements and stream flow records, Burbank, D. W. (1981). A chronology of late Holocene glacier fluctuations on Mount Rainier, Washington. Arctic and Alpine Research, are available through the National 13(4), 369–386. https://doi.org/10.2307/1551049 Water Information System (NWIS) Burbank, D. W. (1982). Correlations of climate, mass balances, and glacial fluctuations at Mount Rainer, Washington, USA, since 1850. archive general site (https://waterdata. Arctic and Alpine Research, 14(2), 137–148. https://doi.org/10.2307/1551112 usgs.gov/nwis/sw). Example links to Church, M., & Ryder, J. M. (1972). Paraglacial sedimentation: A consideration of fluvial processes conditioned by glaciation. Geological NF‐1 measurements (https://waterdata. Society of America Bulletin, 83(10), 3059–3072. https://doi.org/10.1130/0016‐7606(1972)83[3059:PSACOF]2.0.CO;2 usgs.gov/nwis/measurements?site_ Church, M., & Slaymaker, O. (1989). Disequilibrium of Holocene sediment yield in glaciated British Columbia. Nature, 337(6206), 452–454. no=12205000&agency_cd= https://doi.org/10.1038/337452a0 USGS&format=html_table_expanded). Cleveland, W. S. (1979). Robust locally weighted regression and smoothing scatterplots. Journal of the American Statistical Association, Climate data from NOAA's nClimDiv 74(368), 829–836. https://doi.org/10.1080/01621459.1979.10481038 data set for Washington State climate Collins, B., & Sheikh, A. (2004). Historical channel locations of the Nooksack River. Report prepared for Whatcom County Public Works division 5 easiest to obtain from the Department (62 pp.). Retrieved from https://www.whatcomcounty.us/2572/Completed‐Plans‐Nooksack‐River NCDC website (https://www.ncdc. Copeland, E. A. (2009). Recent periglacial debris flows from Mount Rainier, Washington, (Doctoral dissertation). Oregon State University. noaa.gov/cag/divisional/time‐series). Costa, A., Molnar, P., Stutenbecker, L. A., Bakker, M., Silva, T. A., Schlunegger, F., et al. (2017). Temperature signal in suspended sediment Topography (2013 lidar for the export from an Alpine catchment. Hydrology and Earth System Sciences Discussions,1–30. Nooksack River) used to estimate Cui, Y., & Parker, G. (2005). Numerical model of sediment pulses and sediment‐supply disturbances in mountain rivers. Journal of channel distances, elevations, and Hydraulic Engineering, 131, 646–656. https://doi.org/10.1061/(ASCE)0733‐9429(2005)131:8(646) slopes is available from the Puget Sound Cui, Y., & Wilcox, A. (2008). Development and application of numerical models of sediment transport associated with dam removal. In M. LIDAR website (https://pugetsoundli- H. Garcia (Ed.), chap. 23Sedimentation engineering: Theory, measurements, modeling, and practiceASCE Manual (Vol. 110, pp. dar.ess.washington.edu/). 995–1020). VA: ASCE, Reston. https://doi.org/10.1061/9780784408148.ch23

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