The Importance of Widespread Temperature Conditions on Breakup Characteristics: The Case of Sagavanirktok River, Alaska, USA

Abstract

Daily average springtime air temperatures from four weather stations distributed along north–south and nearly east–west directions within or on the divide of the Sagavanirktok River watershed in Arctic Alaska were studied and compared with discharge measurements and field observations made from 2015 to 2021 during breakup. The results indicate that under widespread air temperature events, during El Niño, rapid and dynamic breakup can occur (promoting sediment transport along the stream), while during La Niña, slow and thermal breakup can be expected. Due to these climate pattern effects, open channel conditions (i.e., ice-free channels) are reached earlier (18 May 2015) during El Niño and later (7 June 2021) during La Niña.

1. Introduction

El Niño and La Niña are climate patterns that form in the Pacific Ocean near the Equator. El Niño is characterized by above-average surface water temperatures and weaker-than-normal easterly winds [1]. La Niña is characterized by below-average surface water temperatures and stronger-than-normal easterly winds [1]. Both El Niño and La Niña have an effect on basic meteorological variables such as air temperature and rain, which can lead to drought and flooding. The effects on temperate regions are well documented ([1,2,3,4,5,6], among others); the effects on cold regions are not as well documented [7,8,9].

In Arctic Alaska during spring breakup in 2015, the Sagavanirktok River, which flows north from the Brooks Range, overflowed, causing unprecedented flooding near Prudhoe Bay, a major oil field [10].

The Sagavanirktok is a braided river that parallels the Dalton Highway and the trans-Alaska oil pipeline for approximately 160 km, entering the Beaufort Sea near Prudhoe Bay. The Sagavanirktok basin, which is underlain by continuous permafrost, contains three major regions: the coastal plain, foothills, and mountains. The basin has a low hydraulic gradient near the Beaufort Sea (the coastal plain area) and a high hydraulic gradient in the mountains (the Brooks Range). The basin length is approximately 250 km, and the stream length is at least 300 km. The elevation of the Sagavanirktok River basin ranges from sea level to nearly 2500 m above mean sea level (AMSL), with a mean elevation of 785 m AMSL. The basin area is about 13,500 km². The mountain region has two subbasins: Upper Sagavanirktok and Ivishak. These subbasins are located on the west and east sides of the watershed (Figure 1), with areas of 6500 km² and 5200 km², respectively [10].

During the 2015 breakup, floodwaters caused significant damage to the Dalton Highway, the only terrestrial connection between Fairbanks, the closest city 800 km away, and the Prudhoe Bay oil field [10], and the highway was closed to transit for approximately 3 weeks. The damage to infrastructure was significant. As a consequence of this flooding event, a multi-year project was initiated to monitor river and sediment transport conditions along the Sagavanirktok River [11]. Several meteorological stations were installed within the watershed as part of the project. Station locations are shown in Figure 1.

Numerous and complex variables affect spring breakup, including end-of-winter snowpack and aufeis distribution, but the research presented here focused on the influence of El Niño and La Niña: the effects of widespread air temperature conditions on (a) the breakup characteristics and (b) the date of reported open channel conditions. Breakup can be described as rapid and dynamic or slow and thermal. Data on open channel conditions (i.e., when ice influence on the river cross section is negligible and a rating curve can be applied) were gathered from the U.S. Geological Survey (USGS) station 15908000 near the Trans-Alaska Pipeline Pump Station 3 [12]. Specifically, the results of an analysis of air temperatures from stations representing the Sagavanirktok River watershed were compared with field observations and discharge measurements carried out during spring breakups over the life of the project.

2. Methodology

The sum of daily average temperatures for the period May 1–June 10 was calculated at four weather stations located within or on the divide of the Sagavanirktok River watershed. The stations are distributed along an imaginary north–south line (Prudhoe Bay, Sagwon, and Atigun Pass) and a nearly east–west line (Sagwon and ASM2), as shown in Figure 1. Data for the Atigun Pass, Sagwon, and Prudhoe Bay stations were retrieved from the National Centers for Environmental Information [13], and data for the ASM2 station, installed as part of the river monitoring project, were reported by Toniolo et al. [14].

Table 1. Station coordinates and elevation.

Station	Latitude (°)	Longitude (°)	Elevation (m)
Atigun	68.13	−149.48	1463
Sagwon	69.42	−148.69	305
Prudhoe Bay	70.27	−148.57	9
ASM2	68.77	−147.43	945

provides the geographical coordinates and elevation of the stations. As indicated in Table 1, the stations are located at different elevations, representing different physiographic settings within the watershed.

The analyzed record covers the period from 2014 to 2021 to provide antecedent through end-of-project conditions, including the historic flooding of the Dalton Highway in 2015. Two main factors determined the first (May 1) and last (June 10) day of the study period: (a) the temperature conditions before water begins flowing in the river, and (b) the last day of observed spring breakup peak discharge (for the duration of the project).

Performing any fieldwork activity in Arctic rivers, especially during breakup, is difficult and potentially dangerous, involving multiple constraints, as reported by Keech et al. [15]. The Sagavanirktok River is no exception. River access, floating ice, and multiple channels are some of the challenges encountered when working in the area. Discharge measurements reported in the following section were performed following standard gauging procedures, using an Acoustic Doppler Current Profiler (ADCP). Initial measurements (2015 and 2016) were carried out using the ADCP tethered to a boat. In subsequent years, the ADCP was tethered to a helicopter by a sling line [15].

3. Results

A summary of discharge measurements reported previously [10,14] is shown in Figure 2. These measurements were carried out during spring breakup from 2015 to 2021, except for 2020, when no measurements were carried out because of the COVID-19 pandemic. The values reported in Figure 2 indicate that early and high flows happen during El Niño years, while significant flows are delayed considerably during La Niña years.

Table 2. Sum of daily average temperatures (°C) and reported open channel conditions near Pump Station 3.

Station	Year
	2014	2015	2016	2017	2018	2019	2020	2021
Atigun	−56	81	1	61	−6	67	85	−46
Sagwon	46	154	72	−38	−77	81	19	−63
Prudhoe Bay	−16	36	5	−61	−141	−21	−94	−82
ASM2	-	-	-	65	6	198	129	−55
Open Channel	28 May	18 May	25 May	1 June	29 May	23 May	5 June	7 June

shows the sum of daily average temperatures at the analyzed stations, given on a yearly basis, and the initial date when the rating curve was used to report daily discharge by the USGS. An inspection of Table 2 indicates widespread warm temperatures during 2015 and cold temperatures during 2021 in the study area. These figures agree with the available weather data reported by the Climate Prediction Center [16], showing that 2015 and 2021 correspond to El Niño and La Niña, respectively. Data from the years in between show considerable temperature differences between locations within the watershed, demonstrating that warm or cold temperatures vary across its entire expanse. Based on these data, one can expect early, fast, and dynamic breakups during El Niño years (e.g., 2015), and long, slow, and thermal breakups during La Niña years (e.g., 2021). Figure 3 and Figure 4 show flow conditions along the right bank of the Sagavanirktok River during breakup in 2015 and 2021, respectively, approximately 30 km south of Prudhoe Bay. The photographs clearly show markedly different flow conditions (open channel vs. ice covered) on 18 May 2015 and on 18 May 2021, indicating the influence of El Niño in Figure 3 and La Niña in Figure 4.

When air temperature varies significantly within the contributing areas of the watershed, breakup can present different characteristics along the river. Yearly breakup field observations during the 2015–2021 period corroborate the previous statement [14,17,18].

Further analysis of data provided in Table 2 shows the existence of a relationship between the average air temperature within the watershed in a given year and the date of the reported open channel conditions. The coefficient of determination of this relationship improves from R² = 0.49 to R² = 0.63 when the stations at Prudhoe Bay and Sagwon are used in the calculations, indicating that these two stations play an important role in the prediction of open channel conditions due to their physiographic similarities.

For planning purposes, one can expect early, fast, and dynamic breakups during El Niño, when warm temperatures are widespread within the Sagavanirktok River watershed (such as occurred in 2015). This type of breakup could pose risks including damage to infrastructure along the river, especially in the presence of certain combinations of end-of-winter snowpack and aufeis development. Such breakups promote sediment transport processes inside the channel, leading to major channel modifications. Conversely, one should not expect such issues during La Niña events, when cold temperatures are distributed within the watershed (which occurred in 2021).

4. Discussion

In general terms, bed sediment transport occurs when the following two conditions are reached simultaneously: (a) sediment is available on the riverbed, and (b) the bed shear stress created by flowing water is enough to move sediment particles along the bed. In addition, if the initial riverbed is formed by particles with different diameters and the acting shear stress is only capable of moving smaller diameters, an armor layer [19] will develop on the riverbed. Thus, one could expect a reduction in bed sediment transport over time (i.e., the riverbed becomes paved with big particles, which protect the smaller particles beneath the top layer).

During winter months, ice covers the bottom of most Arctic rivers. This ice, known as bed-fast ice, creates a protective layer on the bed sediments. During spring breakup, the following two scenarios are possible:

(1)

During La Niña events, air temperatures are colder, and snow melts slowly within the watershed. Accordingly, water reaching the river is limited, with two results: (a) the water does not have enough energy to break the bottom ice layer, and (b) expected bed shear stress is low. These conditions lead to sparse or nonexistent bed sediment transport.

(2)

During El Niño events, air temperatures are warmer, and snow melts rapidly within the watershed. Rapid snowmelt generates vast amounts of water moving toward and into streams. Such conditions increase the available energy and expected shear stress, resulting in the possibility of freeing large blocks of bed-fast ice. During this process, ice blocks that detach from the river bottom carry frozen sediment with them (Figure 5), a phenomenon known as ice rafting ([20,21,22], among others), destroying any armor layer on the riverbed so that bed sediment particles of multiple grain sizes are available for transport. In addition, breakup during El Niño is distinguished by rapid velocities (with a maximum speed around 5 m/s on the Sagavanirktok River), which create high shear stresses. Substantial bed-sediment transport rates should be expected.

An intermediate scenario could occur during neutral years (i.e., years not affected by El Niño or La Niña). In this scenario, bed sediment loads during breakup should be smaller than the expected loads under El Niño.

In the present case (i.e., the Sagavanirktok River), the available data (Figure 2) indicate that peak flows during El Niño and La Niña occur before the end of May and in early June, respectively.

While developing a specific bed-sediment transport equation for the Sagavanirktok River, Toniolo [11] speculated on the importance of the armor layer and rapid breakups on bed-sediment transport rates but made no direct (or indirect) connection with El Niño or La Niña events. Sediment transport rates could be calculated using the information provided in that article [11]. Despite limited data, the values reported for 2016, 2017, and 2018 breakups in Table 4 of that article correlate well with the argument presented above and warrant further study, since the Arctic continues to warm, possibly resulting in a bed-sediment transport regime shift.

Global warming could reduce peak discharge during breakup. As indicated by Burrell et al. [23], who stated the following: “a reduced snowpack may limit the amount of snowmelt runoff, which could reduce breakup flows and inhibit the occurrence of dynamic breakup events in regions where the entire snowpack melts very quickly in the spring.” Additionally, under a warming scenario, permafrost degradation would induce the growth of the active layer thickness, which would promote an increase in subsurface flows. These subsurface flows would retard the formation of bed-fast ice.

5. Conclusions

An analysis of daily average springtime temperatures from four weather stations within or on the divide of the Sagavanirktok River watershed from May 1 to June 10 during the period 2015–2021, paired with reported open-channel flow conditions and yearly breakup discharge measurements during the period 2015–2021, indicates the following: (a) that breakup could be early, fast, and dynamic under conditions of widespread warm temperatures, which occur during El Niño events, posing risks to infrastructure near the river; (b) that breakup could be long, slow, and thermal under conditions of widespread cold temperatures, which occur during La Niña events, which may not pose risks to infrastructure near the river; and (c) that breakup with varied characteristics could occur with irregular temperatures in the watershed. During periods of widespread warm temperatures, one could expect high sediment transport rates during breakup, which could affect river channel configuration.

Data (see Table 2) support the existence of an intrinsic relationship between average air temperature within the watershed and the date during spring when open channel conditions are reached after breakup.

Findings from this study could lead to a better understanding of the timing (and potentially the magnitude) of sediment transport processes in other rivers located in similar settings.

Future research should look into the role of the following: (a) aufeis (magnitude and distribution along the stream); (b) end-of-winter snow distribution within the watershed (snow depth and snow water equivalent); (c) water levels heading into freeze up; and (d) winter discharge. It is expected that the knowledge gained from these tasks would improve the prediction of main breakup characteristics.