Geological Society of America Bulletin, published online on 9 March 2016 as doi:10.1130/B31427.1

Controls on gravel termination in seven distributaries of the Selenga Delta Controls on gravel termination in seven distributary channels of the Selenga River Delta, Baikal basin, Russia

Tian Y. Dong1,†, Jeffrey A. Nittrouer1, Elena Il’icheva2, Maksim Pavlov2, Brandon McElroy3, Matthew J. Czapiga4, Hongbo Ma1, and Gary Parker4,5 1Department of Earth Science, Rice University, MS-126, 6100 Main Street, Houston, Texas 77005, USA 2Laboratory of Hydrology and Climatology, V.B. Sochava Institute of Geography, Siberian Branch Russian Academy of Science (SB RAS), Ulan-Batorskaya Street, Irkutsk, 664033, Russian Federation 3Department of Geology and Geophysics, University of Wyoming, 1000 East University Avenue, Laramie, Wyoming 82071, USA 4Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, 205 North Mathews Avenue, Urbana, Illinois 61801, USA 5Department of Geology, University of Illinois at Urbana-Champaign, 605 East Springfield Avenue, Champaign, Illinois 61820, USA

ABSTRACT coarse sediment cannot be transported to the effects of tectonic events in terms of their influ- axis of the Baikal Rift basin. The distribution ence on the distribution of sediment, is therefore The Selenga River Delta, Baikal, of sediment grain size in deltaic channels, important for understanding part of the strati- Russia, is ~600 km2 in size and contains as related to hydrodynamics and sediment graphic architecture of the Baikal Rift basin. multiple distributary channels that receive transport, plays a critical role in influencing A key factor in understanding sediment trans- varying amounts of water and sediment stratigraphy, because the sustained tectonism port in the Selenga Delta, and the influence of discharge. The delta is positioned along the leads to high preservation potential of the sediment dispersal on fluvial-deltaic stratigra- deep-water (~1600 m) margin of Lake Bai- delta topset sedimentary deposits. Therefore, phy, is evaluating the physical controls on spa- kal, a half-graben–styled rift basin, qualify- the Selenga River Delta provides an oppor­ tial transitions in bed material from gravel to ing it as a modern analogue of a shelf-edge tunity to explore the interactions between sand. For alluvial river channels, the gravel-sand delta system. This study provides a detailed modern deltaic sedimentation processes and transition is relatively well studied based on field survey of channel bed sediment com- tectonics that affect the production of basin observations, physical experiments, and theory position, channel geometry, and water dis- stratigraphy. (e.g., Dietrich and Whiting, 1989; Sambrook charge. The data and analyses presented Smith and Ferguson, 1995, 1996; Knighton, here indicate that the Selenga Delta ex­hibits INTRODUCTION 1999; Frings, 2011; Venditti and Church, 2014), downstream sediment fining over tens of all of which have been leveraged to inform kilometers, ranging from predominantly This study examined the sedimentary dynam- numerical models (e.g., Cui and Parker, 1998a, gravel (coarse pebble) and sand near its apex ics occurring in part of a rift-basin margin by 1998b; Cui, 2006). However, the dispersal of to silt and sand at the delta-lake interface. We evaluating how sediment transport processes gravel in fluvial-dominated deltaic systems, and developed an analytical framework to evalu- and tectonics influence deltaic stratigraphy. It is more specifically how the distribution of sedi- ate the downstream elimination of gravel known that the preservation potential of fluvial- ment size relates to spatially variable bed shear within the multiple distributary channels. deltaic stratigraphy is closely related to tectoni- stress conditions common for most deltas and The findings include the following. (1) The cally driven subsidence, and this interplay may the production of basin stratigraphy, has not Selenga River Delta consists of at least eight be evaluated by the ratio of channel to tectonic been addressed. The Selenga River Delta is a orders of distributary channels. (2) With in- time scales (Kim et al., 2010; Straub et al., 2013). mixed sand-gravel fluvial-dominated lacustrine creasing channel order downstream, chan- In essence, if the tectonically induced sub­ system at the margin of the Baikal Rift basin, nel cross-sectional area, width-depth ratio, sidence rate is faster than the channel reworking Russia (Fig. 1). The delta is known to possess water discharge, boundary shear stress, rate on the delta topset, it is possible to imprint a a significant decrease in median grain size from and sediment flux systematically decrease. significant fraction of the subaerial deltaic land- gravel to silt over a relatively small downstream (3) The downstream elimination of gravel in scape into the subsurface stratigraphy, because distance (tens of kilometers; Il’icheva, 2008; distributary channels is caused by declining tectonic events lower portions of the delta topset Il’icheva et al., 2014), and so the Selenga Delta boundary shear stress as a result of water below the active reworking depth of the later- provides an opportunity to pursue the following discharge partitioning among the bifurcat- ally sweeping distributary channels (Kim et al., questions: (1) How is gravel spatially distributed ing channels. (4) Over longer time scales, 2010; Shchetnikov et al., 2012). As a result, in a across the subaerial portion of a delta, where dis- gravel is contained on the delta topset due to tectonically dominated system, the morphologi- tributaries bifurcate over many orders? (2) What frequent and discrete seismic events that pro- cal processes of the delta topset play a key role physical processes influence the spatial variabil- duce subsidence and accommodation, so that in constructing basin stratigraphy. Deciphering ity of channel bed grain size? (3) What is the the sedimentary processes of the Selenga River influence of delta hydrodynamics­ and grain-size †td10@​rice​.edu Delta topset, and specifically evaluating the variability on channel geometry, distribution

GSA Bulletin; Month/Month 2016; v. 128; no. X/X; p. 1–16; doi: 10.1130/B31427.1; 14 figures; 3 tables.; published online XX Month 2016.

GeologicalFor Society permission of to America copy, contact Bulletin [email protected], v. 1XX, no. XX/XX 1 © 2016 Geological Society of America Geological Society of America Bulletin, published online on 9 March 2016 as doi:10.1130/B31427.1

Dong et al.

Lake 60° N 70° E Baikal 120° E Depth(m) Russia 55° N B A High : 0 Lake Low : –1650 ± Baikal 031,600 ,200 6,400 Kilometers Northern 70° E 120° E Basin

Selenga River Delta

1 54°N 50° N

Elevation(m) 2 High : 3539

Low : 439 3 Saddle Khangai Central Basin Mountains 065 130 260 Kilometers

100° E 105° E 110° E Selenga River Delta Irkutsk Ulan-Ude Southern Basin

0155 10 220 Kilometers 100°E 105°E 51°N C

Figure 1. (A) Digital elevation model (DEM) of the three major sediment and water trib- utary systems of the Selenga basin. A black outline shows the Selenga River drainage basin. Inset map shows the location of in Asia. (B) Bathymetric map of Lake Baikal. The lake is located in southeastern (see inset map). Selenga River Delta (highlighted in the black box) deposits cre- ate a bathymetric saddle that separates the Southern and Central basins of Lake Baikal. (C) Elevation profiles of three major tribu- tary channels with average slopes indicated.

2 Geological Society of America Bulletin, v. 1XX, no. XX/XX Geological Society of America Bulletin, published online on 9 March 2016 as doi:10.1130/B31427.1

Controls on gravel termination in seven distributaries of the Selenga Delta of water discharge, and shear stress? Answer- E ing these questions will provide a thorough understanding of the morphodynamics of the E Selenga Delta. E N

BACKGROUND N

Lake Baikal

The receiving basin of the Selenga River Delta, Lake Baikal, is located in southeastern Siberia, Russia (Fig. 1A). Lake Baikal, formed by a half-graben–styled rift basin, extends over 700 km in length, has an average width of 60 km, and has a maximum depth in excess of E

1600 m (Scholz and Hutchinson, 2000). These N dimensions render Lake Baikal the largest fresh- N water lake in the world in terms of volume. Due to its high-latitude location, Lake Bai- E kal’s water level is relatively insensitive to evap- E oration, as it is estimated that only 13% of its total outflow volume is lost to this mechanism (Colman, 1998). The remaining 87% is con- Figure 2. Regional tectonic map of the Baikal Rift. Lake Baikal contains numerous active trolled by the outlet to the Angara River (Fig. 2). normal faults striking NE-SW (modified from Lunina et al., 2012). The Selenga River Delta Lake level is interpreted to be relatively insensi- is located in the dashed black box. tive to climate variation and instead is controlled primarily by tectonic forcing on 100 k.y. time scales (e.g., Colman, 1998; Scholz et al., 1998). mentation, have created a bathymetric ­saddle in where H is the average flow depth, and S is the front of the Selenga River Delta (Fig. 1; Scholz average water surface slope that would prevail Baikal Rift and Selenga Delta and Hutchinson, 2000). This bathymetric high in the absence of the backwater, which may be separates the South and Central basins in Lake approximated by the bed slope upstream of the The eastern and western shores of Lake Bai- Baikal (Fig. 1). backwater zone (Paola and Mohrig, 1996). In kal are controlled by a series of massive normal The Selenga River Delta is located at the the backwater region, the divergence between faults, oriented from southwest to northeast, southeastern shore of Lake Baikal, where the water surface slope and channel bed slope pro- expressed in more than 3000 m of relief from Selenga River enters the lake approximately duces an increase in flow depth (H) progressing the lake bottom to the topographic mountain normal to the rift axis (Figs. 1B and 2). The downstream. However, during higher-discharge highs (Scholz et al., 1998; Delvaux et al., 1999; Selenga River is the largest source of sediment events, elevated river stage increases flow depth Logachev, 2003). The western margin of Lake and water entering Lake Baikal. Covering ~558 and water surface slope in the backwater region, Baikal also outlines the eastern margin of the km2, the modern Selenga River Delta is one of therefore producing a region of uniform flow Siberia craton (Fig. 2). The Baikal Rift system the largest freshwater deltas in the world (Col- that may extend downstream toward the receiv- propagates northeastward along the craton and man, 1998; Scholz et al., 1998; Gyninova and ing basin. Thus, high water discharges tend to terminates against the Aldan Shield, where, as Korsunov, 2003; Il’icheva et al., 2014). mitigate backwater effects (Fig. 3). This is espe- evidence, ages of sediment cores in the south- cially true for the Selenga River Delta, where ern basin are significantly older than those in the Backwater Hydrodynamics relatively steep water slopes during flood events northern basin (Fig. 1; Logachev, 2003; Jolivet (~3.1 × 10–4) limit L to only a few kilometers As a river approaches its receiving basin, its B et al., 2013). Two mechanisms are hypothesized from the outlet (Table 1; Fig. 1C). hydrodynamics and sediment transport proper- for the initiation of the Baikal Rift: (1) a rising Backwater effect–induced nonuniform flow ties are influenced by a backwater effect, char- asthenosphere plume, oriented along the rift axis in river deltas has an important influence on acterized by downstream decelerating flow that initiated crustal thinning (e.g., Logatchev boundary shear stress and therefore sediment velocity. This is typically represented by an and Zorin, 1987; Gao et al., 2003); or (2) the transport capacity. Consider the relationship M1 water surface profile, where the river water India-Eurasia plate collision, which created an between flow velocity (u) and boundary shear surface profile asymptotically approaches the eastward extrusion of the Amuria plate, caus- stress ( ), provided by: relatively constant water surface elevation of the t ing the Baikal region to extend northeastward receiving basin. This can be calculated based on (Fig. 2; Petit et al., 1996; Lesne et al., 1998). τ=ρC u2, (2) the gradually varied flow equation (Chow, 1959; f Seismic imaging indicates sediment thickness Fig. 3). The characteristic backwater length (L ) of 4–5 km in the South Baikal basin, and up to B where is the fluid density, and C is a dimen- is measured upstream of the river-basin inter- r f 7.5–10 km along the front of the Selenga Delta sionless friction coefficient that accounts for face and can be approximated using: (e.g., Hutchinson et al., 1992; Logachev et al., form drag (e.g., Parker, 2004). Because bound- 2000). These thick sedimentary packages, along H ary shear stress is related to sediment trans- L = , (1) with underlying bedrock highs plus delta sedi- B S port capacity, the rate and size of transportable

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475 was made, specifically where the channel nar- moderate water discharge surface profile rowed to less than 50 m, and only two samples flood water surface profile channel bed profile were collected near the outer and inner banks. 470 The final channel transect occurred where the distributary channels terminated at Lake Bai- Receiving Basin 465 kal, defined as the point at which the subaque- H,flood ous expression of channel banks constructed of H,moderate sediment ceases. A global positioning system L ,flood flow 460 b (GPS) coordinate was taken at each sampling

L ,moderate flow location using a handheld Garmin GPSmap b

Elevation above sea level (m) 62s and a GPSmap 64s, both with GLONASS 455 H,basin capability. Downstream distance between each sampling transect was 2.5–4 km. 450 0102030405060 Distance (km) Bank-Line Survey

Figure 3. Water surface profiles produced from the backwater equation, for moderate and Channel bank-line surveys were conducted flood conditions, modeled using inputs representing the Selenga River conditions. Note at the same transects at which the channel bed the divergence between water surface profiles, with respect to water discharge and the sediment samples were collected (Fig. 4). The channel bed profile, at the transition to the backwater region. Reduced sediment trans- bank-line survey procedures consisted of mea- port capacity occurs in the backwater reach, and this region is shortened downstream suring both banks of each transect, i.e., opposite during flood conditions. bank lines, and included: (1) evaluating the mor- phological condition of the bank line, i.e., erod- ing or accreting, (2) measuring the bank-line sediment decrease nonlinearly with reducing coincide with a 2 yr flood event, and (3) grain elevation above the local water surface eleva- fluid flow velocity. Based on these relation- size of channel bed sediment. These data were tion, (3) measuring the water depth of the chan- ships, where boundary shear stress and sedi- augmented during a second campaign in 2014, nel ~1 m from the bank line, and (4) collecting a ment transport capacity decrease in the deltaic with additional data that included measurements sediment sample from the bank. backwater region, there is also an expectation of bank-line elevations and samples of bank-line Erosional bank lines were identified as those of downstream decreasing bed material grain sediment, from both major and minor distribu- locations where sediment blocks were detached size (Nittrouer, 2013). Therefore, the Selenga tary channels. and collapsing into the river channel (Fig. 5). River Delta represents an ideal case study to Commonly, 5–8-m-tall willow trees and other explore the effect of backwater hydrodynamics Channel Bed Sediment Sampling shrub-like vegetation had fallen into the channel in term of influencing gravel-sand transitions in due to erosion. Bank-line height above adjacent a fluvial-deltaic­ system. Channel bed sediment samples were col- water was measured from the flat surface that lected in both major and minor distributary demarcates the top of the bank to the water sur- FIELD AND LABORATORY METHODS channels (Fig. 4) using a Russian-made sampler face. Water depth was measured adjacent to the capable of collecting sediment from the bed sur- bank line using a survey rod. Sediment samples Two Russian-American collaborative field face up to 5–8 cm in depth. Channel bed sedi- were collected from below the reworked (vege­ expeditions were undertaken on the Selenga ment sampling transects began at the expedition tated) soil layer. Where present, the thickness of River Delta to investigate topset morphology base camp in the fluvial portion of the Selenga the soil layer varied, with a maximum depth during the summers of 2013 and 2014; measure- River, i.e., just upstream of the delta apex of half a meter. ments included sampling the channel bed and (Fig. 4). Three samples were collected along bank-line sediments, and surveying the channel each transect, situated to equally partition the Bathymetry Collection bathymetry and bank-line elevations. Data from channel: near the outer bank, the channel center, field work in 2013 specifically included: (1) delta and near the inner bank. This sampling scheme A Lowrance HDS-7 Gen. 2 fish finder was distributary channel widths and depths, to evalu- was designed to capture grain-size variation used to measure water depth and to image ate channel geometries, (2) water discharges in across the channel, including at different water channel bottom structure. The fish finder was delta distributary channels, which happened to depths. One exception to this measuring scheme equipped with two sonar heads, consisting of dual-frequency (50/200 kHz), downward-

TABLE 1. APPROXIMATED BACKWATER LENGTH FOR THE SURVEYED DISTRIBUTARY CHANNELS looking, single-beam sonar and a dual-fre- Measured average Approximated quency (455/800 kHz) sidescan sonar. These

channel depth, H backwater length, LB two devices were mounted to the stern of an Distributary channels (m) (km) ~2.5-m-long inflatable rubber boat. Boat speed Lobanovskaya 1.9 6.3 was restricted to 9–15 km/h to keep the sonar Levoberezhnaya 2.1 6.9 Galutay 1.8 5.9 head well below the waterline to produce qual- Kazanova 1.6 5.2 ity data. At each bank and bed survey site, a Kharauz 2.7 8.8 Sredneustie 1.4 4.5 cross-channel bathymetric transect was col- Kolpinnaya 1.2 4.0 lected to measure water depth and bed struc- Average 1.8 5.9 ture. Longitudinal profiling of the channels was

4 Geological Society of America Bulletin, v. 1XX, no. XX/XX Geological Society of America Bulletin, published online on 9 March 2016 as doi:10.1130/B31427.1

Controls on gravel termination in seven distributaries of the Selenga Delta

system (e.g., Chow, 1959). As previously men- 106°20′E 106°40′E tioned, high discharge events result in short- ^_Camp *# ± ened backwater lengths, with estimated values Water Discharge Stations ! Sampling Locations of only a few kilometers for the Selenga Delta Surveyed Channels Proval Bay distributary channels. In addition, the fringe of 2013 Bathymetry Lines the delta is composed of sub-aqueous channel banks which allow for a hydraulic connection 52°20′N between channel and floodplain during both 52°20′N ! ! ! low and flood water discharge conditions. This ! ! ! ! factor affects the lateral diffusion of the water !! ! ! ! ! ! ! ! ! ! ! surface slope and prevents stacking of the water ! ! ! ! ! !! !! surface, also known as the M2 water surface ! ! ! ! ! ! ! ! *# profile. Therefore, much of the delta channel *#! ! ! ! ! !*# *# network is approximated to possess uniform ! *# ! ! ! !! *# ! flow conditions during moderate and floodwater ! *#! ! ! ! ! ! discharge conditions, and so application of the ! ! ! ! ! !*# *#! !! !! Manning’s formulation is acceptable for major- !# !! !! * ity of the delta (Table 1; Fig. 3). ! ! !! 52°10′N Manning’s formulation is expressed as: 52°10′N ! ! 1 2/3 1/2 ^_!*#! u = n R S , (3)

042 81216 where u is the mean velocity, and R is hydraulic Kilometers A radius, equal to , where A is cross-sectional 106°20′E 106°40′E P flow area, and P is the wetted perimeter, and n Figure 4. Map showing the location of survey transects, indicated by the black circles, which is the Manning’s roughness coefficient. Here, represent locations where channel bed and bank material sediment samples, bank elevation Manning’s n is set equal 0.035 for the sand measurements, and channel cross-sectional bathymetry data were collected. The white star channels, which is a fairly typical surface indicates the location of the expedition base camp, which represents the Camp datum in sub- roughness value for open-channel flow (Chow, sequent figures. White lines indicate the longitudinal bathymetric transects collected from 1959). In addition, water surface slope S was the Lobanovskaya and Selenginskya sector, in the eastern and western sectors of the delta, estimated for individual distributary channels respectively. White triangles show locations where water discharge stations are located. using National Aeronautics and Space Adminis- tration (NASA) Shuttle Radar Topography Mis- sion (SRTM) data in 2000 (Werner, 2001 and also conducted. These surveys were obtained Based on visual inspection, samples consist- Rabus et al., 2003). To account for the channel by navig­ating­ the vessel downstream to inter- ing of primarily sand and gravel were analyzed bed sediment grain-size variation over the delta, cept the bank lines at 45°, so that the track using the CAMSIZER, and samples containing an additional evaluation of Manning’s n was lines turned 90° at the bank line. Downstream mostly mud and very fine sand were analyzed considered based on the work by Parker (1991) bathymetry surveys were performed in two by the Mastersizer. for mixed sand and gravel channels: major distributary channels: the Lobanovskaya 1 g channel in the eastern sector of the delta, and ANALYTICAL FRAMEWORK FOR =αr , ks ≅ nk Ds90, (4) n k1/6 the Selenginskya channel in the western sec- SEDIMENT TRANSPORT s tor of the delta (Fig. 4). These two distributary where ar is a constant with a value typically channels partition up to 80% of the total water An analytical framework was used to deter- between 8 and 9. Parker (1991) suggested a and sediment load of the Selenga River and mine whether water discharges for Selenga value of 8.1 for gravel-bed rivers. Roughness therefore play major roles in setting hydraulic River Delta distributary channels exerted bed height (ks) is constrained as a function of the and sediment transport properties of the delta shear stress sufficient to initiate motion of the dimensionless numbe nk, where nk = 2, and

(Il’icheva, 2008; Il’icheva et al., 2014). pebble-size sediment. Water discharge data surface sediment size (Ds90) is the size that is were collected at 13 different locations from coarser than 90% of the bed material. Sediment Data Processing July through September 2012–2014, which are Assuming a rectangular channel, where H is periods that coincide with moderate to high much smaller than channel width (B), Manning’s Bed and bank sediments were analyzed in water-discharge conditions. However, these sta- formulation reduces to the following form: the Sedimentology Laboratory at Rice Univer- tions did not spatially overlap with most of the 2/3 1 A 1/2 sity. Grain-size distribution for each sample was surveyed transects (Fig. 4). In order to estimate u = S . (5) n B2/3 measured utilizing a Malvern Mastersizer 2000 water discharge and stress conditions for all of and Retsch Technology CAMSIZER. The for- the channels and transects on the delta topset, Multiplying Equation 5 by cross-sectional area mer instrument is best for measuring mud and Manning’s formulation (e.g., Chow, 1959) was (A) yields a relation for discharge (Q) at each 5/3 very fine sand, and the latter instrument is best applied. Manning’s formulation is well known 1 A 1/2 survey transect, where Q = S . for measuring sediment coarser than 63 mm. for describing uniform flow in an open-channel n B2/3

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A B

1m

2m

C D

0.5m 0.75m

Figure 5. Photographs from the 2013 and 2014 summer field expeditions, illustrating the morphology of eroding bank lines.

We compared measured floodwater dis- After calculating flow properties on the tational acceleration, and D is the characteristic charges, based on the network of stations on the delta topset (Eqs. 3–5), boundary shear stress grain size of the bed material (e.g., median size). delta, to estimated water discharge values pro- is related to sediment transport using a semi- Equation 7 was applied to sand-bed chan- duced by Manning’s formulation for each of the empirically defined relationship to calculate nels to calculate bed-load sediment flux at each

13 water discharge measuring stations. bed-load flux (q*b) per unit channel width (B), survey transect. However, in the Selenga River Estimated water discharges produced from following the Meyer-Peter and Muller bed- Delta, channel bed sediment grain size varies, Manning’s formulation compare favorably to load transport equation modified by Wong and consisting of gravel and sand. Therefore, an the measured water discharges (Fig. 6), thus Parker (2006): additional transport framework must be consid- providing validation for the use of this analytical ered for channels beds composed of a mixture 3/2 framework to calculate water discharge. There- q∗b = 3.97(τ∗−0.0495) . (6) of sand and gravel. In this condition, a surface- fore, Equation 5 was used to estimate mean based transport model was used (Wilcock and water velocities and discharges for all surveyed Here, t * is the nondimensional boundary Crowe, 2003): transects. shear stress (Wong and Parker, 2006). In addi-  7.5 Flow velocity is related to boundary shear tion, q is the dimensionless Einstein measure 0.002Øi ,Øi < 1.35 *b  stress by: = C u2 (Eq. 2), where C is con- of the bed-load transport rate, which is related to W ∗ 4.5 . (8) t r f f i =   0.894 14 1− ,Øi ≥ 1.35 strained by the Manning-Strickler formulation, the volumetric transport rate per unit width qb as:  0.5  Øi  which defines a relationship between average  q flow depth (H) and the roughness height (k ) q = b , (7) Here, W* is a dimensionless bed-load trans- s ∗b DRgD i over a flat surface without bed forms (Parker, port rate associated with size Di in a sedi- 1/6 −1/2  H where R is the submerged specific gravity of ment mixture characterized by multiple grain 1991), where C f =αr .  ks  sediment (1.65 for standard quartz), g is gravi- sizes i = 1…n; here, n = 8. More specifically,

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Controls on gravel termination in seven distributaries of the Selenga Delta

4 the downstream distance of gravel elimination, 10 e where the bed transitions from mixed sand and 2 R =0.84 gravel to a sand bed, is variable among the differ- )

–1 ent distributary channels (Fig. 7B).

s 1:1 Lin

3 There are channel transects with samples that

(m possess median grain sizes of both gravel and 3 10 sand, depending on the cross-sectional position of the sample within the channel. For example, sandy samples within the gravel-sand zone typi- cally occur near the inner bank of the channel, and gravel is typically found in the thalweg, 102 demonstrating cross-channel sediment sorting (Frings, 2011). The distribution of bed materials was measured for all of the distributary chan- nels, and the data were used to inform a mecha- nistic model that predicts gravel mobility on the 1 delta topset. 10 , Calculated flood water discharge l

ca Channel Order Q A topological scheme was manually imple- mented to classify the order of distributary 0 10 channels based on channel bifurcations (Hack, 0 1 2 4 10 10 10 103 10 1957; Voropay et al., 2013; Il’icheva et al., Qmea, Measured flood water discharge(m3 s-1) 2014). As shown in Figure 8A, the Selenga River mainstream is classified as a first-order Figure 6. Measured floodwater discharge and calculated floodwater discharge, using channel near the Camp datum. The system then Manning’s formulation. bifurcates downstream into two second-order channels; these two second-order channels then bifurcate into four third-order channels, and so Rgq ∗ bi downstream. Additionally, as a result of decreas- on. Based on this classification, eight channel Wi = 3 , where qbi is the volumetric trans- Fiu∗ ing sediment transport capacity, fining of the bed orders were identified. The delta channel sys- port rate per unit width for grain-size range i, Fi material grain size is expected. The analytical tem is complex and includes both branching is the fraction of grain size on the bed surface framework presented here links fluid flow and and merging. It is assumed that, if the channel in grain size range i, and u* is shear velocity sediment transport to evaluate the downstream branches into two lower-order channels but 0.5 defined by u* = (t/r) . variability of sediment size and discharge for a merges downstream without significant drain- τ range of flow conditions in time (i.e., low dis- age or merging in between, then the channel Furthermore, Øi = , where t is boundary τri charge to flood events) and in space (i.e., chang- order does not change. Bifurcation produces shear stress. Here, tri is a reference shear stress ing discharge due to bifurcating channels). increasing channel order downstream (Fig. associated with the ith grain size, specified as 8B), and the highest-order channels tend to b τri  Di  RESULTS be concentrated on the eastern portion of the =   , where Dsm denotes the surface τrm Dsm delta, which also presently receives the largest geometric mean size of the bed sediment, and Channel Bed Sediment Distribution proportion of Selenga River water discharge trm denotes the reference shear stress associated (Il’icheva et al., 2014). In addition, this region with this size. The exponent b is given by the Bed material samples indicate that character- of the delta empties into Proval Bay, which was 0.67 produced by the instantaneous subsidence of relation, b . istic bed sediment size varies from gravel (coarse = 2  Di  pebble) near the delta apex (i.e., Camp datum) to ~200 km of delta by ~3 m during a 7.5 magni- 1+ exp 1.5−   Dsm fine sand and silt at the delta-lake interface, dem- tude earthquake event in 1862 (Vologina et al., onstrating that the median grain size of bed mate- 2010). This produced an increase in local chan- We multiplied the computed values of bed- rials varies three orders of magnitude over a dis- nel slope, which attracted distributary channels load transport rate per unit width from the tance of ~35 km. Notably, the extent of the mixed from the central sector of the Selenga River, Wong-Parker and Wilcock-Crowe relations by sand-gravel region starts from Camp datum and therefore increasing the contribution of water unit channel width (B) to calculate volumetric ends ~22 km downstream, indicating that gravel and sediment to the eastern portion of the delta.

sediment discharge (Qb) for each survey transect content declines downstream and eventually van- In fact, since this earthquake event, the active on the delta topset, ishes well upstream of the lake (Fig. 7A). Based depocenter of the delta has switched from the on the grain-size data, we mapped channel bed central sector to the eastern sector (Il’icheva Q = q B. (9) b b sediment distribution to illustrate the spatial vari- et al., 2014). Moreover, due to the complexity Where a channel bifurcates in a deltaic set- ation of sediment size on the delta topset (Fig. of the bifurcating channels on the delta, there ting, boundary shear stress decreases, and there- 7B). Mixed sand and gravel channels are con- is a spatial overlap between different channel fore bed material transport rates should decrease centrated near the center of the delta. Note that orders with respect to distance downstream of

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A B 106°20′E 106°40′E

2 ^_ Camp ± 10 Sand-Bed Channels d10 Mixed Gravel&Sand Channels d50 1 52°20′N 10 d90 52°20′N

Reference Gravel, D=3.6mm 0 10

–1 10 Grain size(mm)

–2 10 52°10′N 52°10′N ^_ –3 10 042 81216 0510 15 20 25 30 35 40 Kilometers Distance downstream of camp (km) 106°20′E 106°40′E

Figure 7. (A) Grain-size data from the channel bed sediment collected from the seven Selenga Delta distributary channels. Sediment size varies from coarse pebble gravel (10–35 mm) upstream to silt and clay at the delta-lake interface. Median grain diameter decreases by three orders of magnitude over 35 km. Note that gravel is eliminated from the channel bottom at a distance 15–25 km upstream of the lake outlet. The dashed line indicates the reference gravel size (3.6 mm) used to compute gravel transport stage. (B) Planform map of the channel bed sediment composition. White lines illustrate the regions consisting of mixed sand and gravel channels. Black lines illustrate sand-bed chan- nels with no gravel. Notice that the downstream distance for the mixed gravel-sand to all-sand bed varies between distributary channels.

A 106°20′E 106°40′E ± B

40 52°20′N 25%–75% 52°20′N N (km) 35 Median 30 1%–99% ^_ Camp Order_8 25 Order_7 Order_6 20 Order_5 Order_4 15 Order_3 10 Order_2 Order_1 5 Distance downstream of camp

52°10′N 0 52°10′N N 12345678 Channel order ^_

042 81216 Kilometers

106°20′E 106°40′E

Figure 8. (A) Channel order map of the delta. The first-order channel is the main Selenga River channel, and the increasing order of chan- nels occurs due to bifurcation. The highest ordering of channels is located on the eastern portion of the delta, which coincides with the delta region that receives the greatest proportion of Selenga River water and sediment discharge. (B) Channel order and downstream distance from the camp datum.

8 Geological Society of America Bulletin, v. 1XX, no. XX/XX Geological Society of America Bulletin, published online on 9 March 2016 as doi:10.1130/B31427.1

Controls on gravel termination in seven distributaries of the Selenga Delta the Camp datum (Fig. 8B). This arises because variable estimates of flow velocity and bound- several long sediment cores adjacent to the delta channels do not bifurcate symmetrically across ary shear stress based on Equations 2 and 5, as indicate that gravel does not reach the axis of the the delta. previously mentioned. Baikal Rift (e.g., Hutchinson et al., 1992; Scholz et al., 1998; Müller et al., 2001; Colman et al., Channel Geometry Sediment Flux 2003; Logachev, 2003; Sapota et al., 2006). From the perspective of understanding the stra- Bathymetry data and bank-line elevation Volumetric sediment flux for each survey tigraphy of this high-gradient dispersal system, it measurements were combined to calculate transect was calculated for bankfull flow condi- is important to investigate the cause of sediment the bankfull cross-sectional flow area at each tions using Equations 6–9. The spatial distribu- fining over the Selenga Delta topset. survey transect. Overall, channel cross-sec- tion of estimated sediment flux shares a simi- Backwater hydrodynamics reduce bound- tional area decreases downstream, as would lar trend with estimated shear stress, because ary shear stress in delta environments, and the be expected, corresponding to the increasing they are closely related. The calculations indi- spatial extent of this hydrodynamic condition channel order. Branching and remerging of the cate that sediment flux decreases downstream is estimated using the characteristic slopes and distributary channels create local increases in with increasing bifurcation number (Figs. 9K flow depths (e.g., Chow, 1959; Parker, 2004; channel area, particularly in the second-order andL9). The center of the delta topset experi- Nittrouer et al., 2011; Nittrouer et al., 2012). For channels (Figs. 9A and 9B). Channel widths ences greater volumetric sediment flux due to the Selenga system, however, we can rule out were extracted from the bathymetry data, and the large characteristic bed grain size and high backwater conditions as the cause of the gravel these values were checked using high-reso- boundary shear stress. Summing the calculated elimination from the transport system, because lution satellite imagery of the delta topset. bankfull sediment discharges for each chan- the backwater length during morphodynami- By combining width data and average chan- nel order resulted in a downstream decrease in cally active discharge conditions (i.e., flood to nel depths determined from the bathy­metric sediment flux with increasing channel order, bankfull) is quite short. Consider the seven data, the width-depth aspect ratios (B/H) were therefore implying that some of the sediment major distributary channels: Based on mea- calculated for the survey transects. These must be depositing (Fig. 10). Yet, the large dif- sured depths, and using a slope estimated from width-depth ratios are largest for the first- and ferences in sediment discharges among sixth-, the delta apex upstream to the Mostovoy water second-order channels, while third- to eighth- seventh-, and eighth-order channels are actually discharge station (~100 km upstream, where S = order channels maintain a lower and relatively due to the fact that not all of the seventh- and 3.1 × 10–4), the average estimated backwater constant ratio (Figs. 9C and 9D). eighth-order channels were surveyed; therefore length for the Selenga distributary channels is the sum of measured seventh- and eighth-order ~5.9 km (Table 1). Moreover, for each distribu- Water Discharge channels underestimates the total sediment dis- tary channel, the backwater distance remains charge. Furthermore, the increase in sediment downstream of the mixed gravel and sand to After constraining channel geometry, water flux between first- and second-order channels sand bed transition (Fig. 11). Therefore, for the discharge was calculated for each survey tran- is due to the fact that calculated bankfull water Selenga River Delta, the decline in sediment sect over the delta for an estimated bankfull discharges provide a maximum for flow condi- transport capacity and downstream fining of flow condition, by which bankfull flow area tions, as discussed already. channel sediment require an alternative expla- was estimated for each survey transect using nation other than a backwater-induced decrease bathymetry transect data and bank-line height DISCUSSION in sediment transport capacity. measurements. Water discharge decreases Application of Equation 2 to field data indi- downstream as the channels bifurcate into Shear Stress Variation on the Delta Topset cates that the bifurcating channel network on smaller-order channels (Figs. 9E and 9F). and Sediment Transport Dynamics the Selenga Delta topset influences the spatial Channel branching and merging within the distribution of shear stress. This is because same channel order can increase flow area The Selenga River is fed by high-gradient cross-sectional flow area, water discharge, and locally, and due to the relationship between tributary channels, with the major ones descend- flow velocity decrease as the channels bifurcate discharge and channel area expressed in Man- ing from mountain ranges located within the and produce higher-order distributary channels ning’s formulation, these local increases in southern drainage basins of Mongolia (Fig. downstream (Figs. 9A–9L). These parameters flow area result in variable estimates of water 1A). The basic depth-slope product (t = rgHS) are related to shear stress (Eqs. 2–5); therefore, discharge. for normal flow conditions can be used to esti- shear stress is also reduced downstream due to mate reach-average bed shear stress (t) for these water partitioning among the bifurcating chan- Flow Velocity and Boundary Shear Stress tributaries. In applying this equation, bed slope nels. Furthermore, because boundary shear is estimated based on various basin slopes S for stress is related to sediment transport capacity, Mean flow velocity and boundary shear stress the system, and it ranges ~3.08 × 10–4 to 2.82 × and sediment flux is also reduced, therefore, were calculated at each survey transect based on 10–3 (NASA SRTM data, including Werner, 2001 shear stress is not sufficient to produce gravel measured channel geometry using Equations 2 and Rabus et al., 2003; Fig. 1C). Using charac- transport for the higher-order channels. For the and 5. The results show that, due to a reduction teristic values of H, the predicted shear stresses Selenga Delta, field data indicate that this occurs in flow velocity, shear stress decreases down- are sufficient to transport gravel throughout the for the third- to sixth-order channels. stream with increasing channel order due to Selenga River basin (Table 2) to the delta apex. It bifurcations (Figs. 9G–9J). The minor variation is interesting that, after thousands of kilometers Downstream Elimination of Gravel in shear stress between third- and fourth-order of gravel transport throughout the Selenga basin, channels may be due to the fact that channel samples collected from the delta indicate gravel To further explore the effects of the chan- branching and merging within the same channel elimination occurs within only tens of kilome- nel networks on sediment transport across the order increase flow area locally, which produces ters upstream of the delta-lake interface. Indeed, delta, we examined the change in boundary

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Dong et al.

4 4 10 10 A B y=635.8e–0.0000401x R2=0.21 3 3 10 10

) Figure 9. (A) Bankfull cross-sec- 2 ) 2

(m tional area and channel ­order. (m 2 2 10 10 (B) Bankfull cross-sectional area and distance downstream 1 1 Bankfull cross-sectional area of the Camp datum. The in-

Bankfull cross-sectional area 10 10 0510 15 20 25 30 35 40 12345678 Channel order Distance downstream of camp(km) crease in channel order and dis- 3 3 10 10 tance coincides with decreasing C D y=88.7e–0.0000236x 25%–75% R2=0.088 cross-sectional area. (C) Width- 2 2 Median 10 10 to-depth ratios and channel 1%–99% ­order. (D) Width-to-depth ratios Outliers 1 1 Survey and distance downstream of the 10 10 Transects

Width-depth ratio Camp ­datum. Width-to-depth Width-depth ratio ratios are large for the first- 0 0 10 10 and second-order channels, and 12345678 0510 15 20 25 30 35 40 Channel order Distance downstream of camp (km) 4 4 they decrease downstream for 10 E 10 F y=896.4e–0.0000659x the third- through eighth-order 2 3 3 R =0.33 channels. (E) Bankfull water ) 10 ) 10 –1 –1

s discharge and channel order. s 3 3 (m

(m (F) Bankfull water discharge 2 2 10 10 and distance downstream. The Bankfull water discharge

Bankfull water discharge increase in channel order and 1 1 10 10 12345678 0510 15 20 25 30 35 40 distance downstream coincides Channel order Distance downstream of camp (km) with decreasing bankfull water 2 2 G H y=–0.000022x+1.32 discharge. (G) Bankfull flow R2=0.39 velocity with channel ­order. 1.5 1.5 ) ) (H) Bankfull flow velocity and –1 –1 s s 1 1 distance downstream of the (m (m Camp datum. The increase 0.5 0.5 in channel order and in dis- Bankfull mean flow velocity

Bankfull mean flow velocity 0 0 tance coincides with decreas- 12345678 0510 15 20 25 30 35 40 Channel order Distance downstream of camp (km) ing bankfull flow velocities.

1 1 (I) Shear stress and channel 10 10 J I y=9.08e–0.000103x order. (J) Shear stress and dis- R2=0.62 tance downstream of the Camp ­datum. Increasing channel 0 0 10 10 (Pa)

(Pa) ­order and distance downstream Shear stress

Shear stress coincides with decreasing shear stress. (K) Volumetric sedi- –1 –1 10 10 ment flux and channel order. 12345678 0510 15 20 25 30 35 40 0 Distance downstream of camp (km) 0 Channel order 10 (L) Sediment flux and distance 10 –1 L –1 K 10 downstream of Camp datum, as 10 –0.000169x –2 y=0.0233e calculated for each survey tran- ) –2 10

) 2 –1 10 R =0.40 s sect. The increase in channel –1 3 –3 –3 s 3 (m 10 10 order and distance downstream –4 (m –4 10 10 coincides with decreasing volu- –5 –5 10 10 metric sediment flux. Bankfull sediment discharge –6 Bankfull sediment discharge –6 10 10 12345678 0510 15 20 25 30 35 40 Channel order Distance downstream of camp (km)

shear stress relative to the downstream elimina- where tn is the boundary shear stress at a survey therefore it provides a minimum critical stress for tion of gravel. Here, we utilized the transport transect, and tcr is the critical shear stress of the initiation of gravel motion. We found this param- stage of gravel (T) to relate critical shear stress smallest D90 gravel size (3.6 mm) measured from eter to have a roughly constant value of critical to boundary shear stress: all the grain-size data (Fig. 7). This reference shear stress, equal to 1.39 Pa, using the estimate grain size captures the spatial variability of gravel of the Shields curve from Parker et al. (2003),

τn sizes. For example, it is equivalent to D10 of the T = , (10) −0.6 τ −0.6 (−7.7 Rep ) cr bed material samples near the delta apex, and τ∗c = 0.5(0.22Rep + 0.06⋅10 ), (11)

10 Geological Society of America Bulletin, v. 1XX, no. XX/XX Geological Society of America Bulletin, published online on 9 March 2016 as doi:10.1130/B31427.1

Controls on gravel termination in seven distributaries of the Selenga Delta

0 within the delta, or gravel should be expected 10 y=0.40e–0.78x to, over time, migrate to the receiving basin. For 2 example, morphodynamic theory and numerical R =0.64 –1 10 models predict that “feeding” gravel to the delta will raise bed slope to produce a sufficient shear stress to transport gravel farther downstream –2 (Hoover Mackin, 1948; Cui and Parker, 1998a;

) 10

–1 Parker, 2004). Time is a critical parameter to s 3 this effect, and time can be estimated by com- (m –3 10 paring the gravel volume required to aggrade and adjust the bed slope necessary to exceed the gravel transport threshold to the flux of gravel –4 10 from the Selenga River. The former parameter (volume required to Calculated bankfull sediment discharge raise bed slope to the gravel transport threshold) –5 10 is estimated based on the relationship between 0 123456789 boundary shear stress and channel slope, i.e., Channel order using the depth-slope product, t = rgHS. Since = at the transport threshold of gravel, the Figure 10. Sum of calculated bankfull sediment discharge for each channel order. In gen- t tcr depth-slope product is modified to estimate the eral, calculated bankfull sediment discharge decreases downstream with increasing channel critical slope necessary to match critical stress order, indicating deposition of sediment. The large difference in sediment discharge among of gravel, defined as , so: = gH , where the sixth-, seventh-, and eighth-order channels is due to the fact that not all distributary b tcr r b the value of is set to the mean critical shear channels bifurcate to the highest-order channels (i.e., channels of order six to eight termi- tcr stress of the measured D gravel (D = 14 mm) nate into the lake). 50 from all the grain-size data, which captures the size range of gravel transported by the Selenga where Rep is the particle Reynolds number, each of the distributary channels spatially coin- River main channel. For this condition, critical RgDD cide with the channel bifurcation points (Fig. stress is equal to 6.4 Pa (Eq. 11) and is the value equal to , and v is the kinematic viscos- v 13). Since T is defined as a ratio of shear stress, used to estimate the slope (b) required to pro- ity of water. If T > 1, the reference gravel size and shear stress varies as a function of the exist- duce gravel transport. (D = 3.6 mm) is transported because bound- ing channel bifurcation network, these results A first-order mass balance framework is used ary shear stress exceeds critical shear stress. support the contention that gravel elimination to estimate the volume of gravel required to If T < 1, then the reference gravel size does within the distributary channels is a result of aggrade the bed and produce the critical slope not move. We determined values of T at every decreasing water discharge. Moreover, the (b) sufficient to exceed the gravel transport survey location for the measured floodwater demise of gravel transport capacity within threshold, for each of the seven major distribu- discharge (July 2013) and also for estimated the bifurcating channel network occurs between tary channels. For simplicity, the volume of bankfull conditions, using the ratio of calculated the third- to sixth-order channels. gravel deposited was estimated using a three- boundary shear stress (Eq. 2) and the reference dimensional geometric configuration (Fig. 14). gravel critical shear stress. Paradox of Arrested Gravel Front The slope calculations consider the distance

For both the July 2013 flood and the estimated (x1) between the Camp datum and the measured bankfull discharge, survey transects that satisfy The transport stage of gravel (T) suggests downstream limit of gravel for each distribu- T > 1 conditions coincided with the measured that the locations where gravel terminates on tary channel. In order to determine the volume locations of the mixed sand and gravel channels the delta topset are fixed in time and space. of gravel required to aggrade the channel beds (Fig. 12), as is expected due to the presence of However, because there is a continuous feed and increase the slope to the threshold bound- gravel in these channels. of gravel from the upstream Selenga River to ary shear stress, a distance of gravel prograda- Figures 12A and 12C show that the observed the downstream delta, and because sediment tion must be defined. In the gravel region of the gravel elimination zone spatially coincides with mass must be conserved, then it stands to rea- delta, the mechanism for shear stress reduction the calculated gravel elimination zone for both son that gravel is either removed from the is the partitioning of water discharge at channel flood and bankfull water discharge conditions. active dispersal system (e.g., buried below the bifurcations (Fig. 13). Therefore, the estimates Furthermore, the gravel elimination zones for morphodynamically­ active topset) somewhere here consider displacing gravel to the near- est downstream bifurcation node with respect

to the current termination location (x2). These TABLE 2. APPROXIMATED SHEAR STRESS OF THE SELENGA RIVER conditions limit the length of slope adjustment Characteristic channel depth Measured τ to the delta topset. Estimated slopes of the dis- Tributary channels (m) average slope* (Pa) tributary channels are indicated by angle a; the Upland reach 1.0 0.00282 27.7 height dimension of the defined volume (y1) is Middle reach 2.0 0.000623 12.2 Lower reach 3.0† 0.000308 9.1 equal to x1Tan(a), and so the volume of gravel *Measured from National Aeronautics and Space Administration (NASA) Shuttle Radar Topography Mission deposited in a channel is V1 – 0.5x1y1B, where (SRTM) data 2000 (Werner, 2001 and Rabus et al., 2003). B is the average channel width. The volume of † Measured at Maloe Kolesovo station, ~40 km upstream of Lake Baikal. gravel needed to increase the slope was calcu-

Geological Society of America Bulletin, v. 1XX, no. XX/XX 11 Geological Society of America Bulletin, published online on 9 March 2016 as doi:10.1130/B31427.1

Dong et al.

subsidence rate, here ranging 6–8 mm/yr. The 106°20'E 106°40'E tectonic time scale (Tt) is therefore computed as Approximated Backwater Length 390–560 yr, which is a similar value compared Mixed Gravel&Sand Channels ± to the observed recurrence time of earthquakes. Surveyed Channels ^_ Camp The time required to aggrade the channel bed, increase slope, and prograde the gravel front 52°20'N Kolpinnaya basinward (T ) is longer than the tectonic time 52°20'N Kazanova Lobanovskaya g scale (Tt), i.e., Tg > Tt. Therefore, the distributary Galutay Sredneustie channels over the Selenga River Delta topset are temporally limited in terms of the ability to Kharauz regrade and disperse gravel downstream. That is, while the slopes of channel beds regrade and produce a stress necessary to transport gravel downstream, the relative rapidity of earthquake- induced subsidence disrupts the dispersal process because gravel must fill the accommodation cre- ated by subsidence before basinward movement. Thus, for the Selenga Delta, gravel is removed Levoberezhnaya 52°10'N 52°10'N from the active transport system due to episodic tectonic events that lower delta deposits below ^_ the depth of laterally sweeping distributary channels. Therefore, gravel is sequestered to the long-term delta stratigraphy. As a result, while 0482 12 16 Kilometers perhaps transient on years to decadal time scales, the gravel front for the Selenga system should be 106°20'E 106°40'E relatively stationary (i.e., held between bifurca- tion nodes, location where channel splits), unless Figure 11. Map demonstrating the mixed gravel and sand channels transitioning to sand the large-scale basin climate pattern or tectonic channels for each of the seven major distributary channels, and the calculated backwater activity adjusts. As mentioned already, however, zones for the channels. The elimination of gravel occurs upstream of the backwater zone there is little evidence to suggest that either cli- estimated for all channels. mate or tectonics have changed significantly over the multimillennial time scales necessary to disperse gravel across the delta topset (e.g., lated as Vg = V2 – V1, where V2 = 0.5(x1 + x2)y2B (Tg) required to fill the volume of gravel (Vg) to Colman, 1998; Scholz et al., 1998). As a conse- and y2 = (x1 + x2)Tan(b). Values were determined aggrade the channel bed to increase channel quence, gravel is trapped on the Selenga Delta for each of the seven distributary channels. slope, and prograde gravel downstream, for the topset, filling local accommodation created by The total volume required to raise the slopes Vg tectonically driven subsidence. Additional strati- seven distributary channels is Tg = , where of the distributary channels to the threshold Qg graphic studies that document changes in gravel Tg = 1050–1700 yr. for gravel transport (Vg) was determined to be displacement could be used to help evaluate the 0.073 km3 (Table 3). Modifying this value by In the Baikal basin, tectonically driven earth- role of internal adjustments of the dispersal sys- 25% accounts for the depositional porosity of quakes induce subsidence on the delta topset tem (e.g., changing number of bifurcations and unconsolidated mixed sand and gravel (Leopold­ with a recurrence time of 350–500 yr and with channel orders), or determine some yet-to-be et al., 1964) and yields a value of 0.055 km3 an average of ~3 m of vertical (downward) identified climate adjustment that could drive the of sediment. It is noted that this is a minimum movement per event (Shchetnikov et al., 2012). gravel front farther basinward. depositional volume because only the storage This magnitude and frequency produce sub­ Two mechanisms have been proposed to pro- capacity in the distributary channels is consid- sidence rates of 6–8 mm/yr over time scales of duce gravel and sand transitions in fluvial sys- ered; in actuality, channels migrate laterally centuries to millennia (Vologina et al., 2010), tems not associated with a bifurcating channel and therefore utilize storage capacity across the and this is a minimum estimate of subsidence network: gravel comminution and base-level delta topset. rate because it does not consider sediment com- change (Cui and Parker, 1998a). It is unlikely

To calculate the minimum time scale required paction. The tectonic time scale (Tt), which cap- that comminution is a major control for the to fill the necessary volume to adjust chan- tures the time scale required for the signal of tec- termination of gravel. The delta is >1500 km nel slopes, the influx of gravel from Selenga tonically driven subsidence to propagate across downstream from its gravel source (Fig. 1A), River main channel must be constrained. Using the landscape and to preserve topset sediment to thus remaining gravel is expected to be resistant Equations 8 and 9, we calculated the flux of all basin stratigraphy, was estimated using the for- to comminution, especially over the relatively −1 observed gravel sizes (Qg) for a flood condition. ∆σ short ~15–20 km distance upstream of the delta- mulation by Kim et al. (2010): Tt = Sx , We implemented two intermittency periods, Ly  lake interface (Sambrook Smith and Ferguson, 10 days and 16 days, to capture the temporal­ where Sx is the average slope of the basin, here 1995; Cui and Parker, 1998a, 1998b). The over- variability of the flood durations (Il’icheva represented by the average channel slope. Ly all picture hypothesized here corresponds more et al., 2014), which produced values of Qg = is the width of the basin, here represented by closely to the arrested gravel front of Cui and 32,000–52,000 m3/yr. The minimum time scale the floodplain width (~20 km), and Ds is the Parker (1998a, 1998b), for which subsidence

12 Geological Society of America Bulletin, v. 1XX, no. XX/XX Geological Society of America Bulletin, published online on 9 March 2016 as doi:10.1130/B31427.1

Controls on gravel termination in seven distributaries of the Selenga Delta

A 106°20′E 106°40′E B ^_ Camp 0.11–0.99 Distribution of T ± T < 1 bankfull discharge 1.00–5.82 T > 1 2 Galutay B) 10 Sand-Bed Channels Kolpinnaya Kazanova Mixed Gravel&Sand Channels Kharauz 52°20′N 52°20′N Lobanovskaya Kolpinnaya Kazanova Lobanovskaya Galutay 1 Levoberezhnaya 10 Sredneustie Sredneustie

Kharauz 0 T=1 10 bankfull discharge Levoberezhnaya T –1 52°10′N 10 52°10′N ^_ –2 042 81216 10 Kilometers 0510 15 20 25 30 35 40 Distance downstream of camp 106°20′E 106°40′E (km)

C 106°20′E 106°40′E D ^_ Camp Distribution of T flood discharge 0.05–0.99 ± T < 1 1.00–4.21 T > 1 Galutay Sand-Bed Channels 2 Kolpinnaya 10 Kazanova Mixed Gravel&Sand Channels Kazanova 52°20′N Kharauz 52°20′N Lobanovskaya Kolpinnaya Lobanovskaya Galutay 1 Sredneustie 10 Levoberezhnaya Sredneustie

Kharauz 0 T=1 10 flood discharge T –1 10

52°10′N 52°10′N Levoberezhnaya ^_ –2 10 0510 15 20 25 30 35 40 042 81216 Distance downstream of camp (km) Kilometers

106°20′E 106°40′E

3 –1 Figure 12. (A) Distribution of gravel transport stage (T) under bankfull condition (Qcal = ~2150 m s ). White circles indicate T > 1, which represent zones of gravel transport. Black circles indicate T < 1, which represent zones of no gravel transport. White channels indicate a mixed sand-and-gravel bed, and black channels indicate sand beds. The observed gravel elimination zone spatially coincides with the calcu- lated gravel elimination zone under bankfull condition. (B) Transport stage of gravel (T) and distance downstream of the Camp datum, cal- culated for bankfull water discharge for the distributary channels. (C) Distribution of gravel transport stage (T) for floodwater discharge 3 –1 condition (Qmea = ~1400 m s ) with the same legend as A. Again, the location of gravel transport spatially coincides with the existence of mixed gravel and sand channels. (D) Gravel transport stage (T) and downstream distance of Camp datum for floodwater discharge, where the observed gravel elimination zone spatially coincides with the calculated gravel elimination zone. drives downstream decline in slope and sedi- Implication for Stratigraphy: dominantly fine material. Data from drill core ment transport rate, and gives rise to a discrete Fate of the Gravel and associated seismic images offshore of the point beyond which all the gravel is buried, and Selenga River Delta confirm this prediction; none is available for downstream transport. The sediment grain-size distribution of the the sediments are reported to be sand and silt In Cui and Parker’s analysis, however, only a Selenga River Delta topset directly impacts the (e.g., Hutchinson et al., 1992; Scholz et al., single channel was considered (with no effect stratigraphy of sediment deposits in the Bai- 1998; Müller et al., 2001; Colman et al., 2003; of channel bifurcation), and subsidence was kal basin. Since the gravel front is relatively Logachev, 2003; Sapota et al., 2006). However, assumed to be continuous rather than associated fixed on the delta topset, the sediment reach- the ages from drill-core data only extend back to with discrete earthquakes. ing the basin trough is expected to be pre- the Pliocene, and the gravel front may migrate

Geological Society of America Bulletin, v. 1XX, no. XX/XX 13 Geological Society of America Bulletin, published online on 9 March 2016 as doi:10.1130/B31427.1

Dong et al. 106°20′E 106°40′E ±

52°20′N 52°20′N

0.11–0.99 1.00–5.82 Figure 13. Map of the gravel ^_ Camp elimination zones, indicated by Order_8 the dashed white lines, and the Order_7 locations of channel bifurca- Order_6 tion. The two spatially coincide Order_5 because shear stress is reduced Order_4 due to water partitioning at Order_3 channel bifurcations. Order_2 Order_1

52°10′N 52°10′N ^_

042 81216 Kilometers 106°20′E 106°40′E

over a longer time as influenced by changing earthquake event would have driven the tempo- the gravel front could prograde. Alternatively, tectonic or climate regimes. Results of numeri- rary cessation of gravel dispersal at this location with relatively short time intervals between cal modeling studies indicate that a gravel front because the tectonism would have triggered an earthquake (subsidence) events, channels may progrades basinward under a “quiet period of avulsion event, thereby relocating the primary not extend relatively far basinward, and the tectonism” (Paola et al., 1992), because the vol- distributary channel to another location on the system would be affected by high-frequency ume of accommodation created by subsidence topset. The basinward extent of a given gravel avulsions. This supposition is supported by the is significantly reduced during such a period, front for a cycle therefore indicates the relative recent study of Reitz et al. (2015), who showed which in turn allows gravel to aggrade the chan- time period between earthquakes: the longer the that avulsion frequency scales with rate of sub- nel bed, increasing channel slope, and prograd- period of reoccurrence, the farther basinward sidence, and by Colman (1998), who identified ing the gravel front basinward. Three separate rifting events are proposed in the Baikal region, each with a different subsidence rate (Logachev, 2003; Petit and Déverchére, 2006). Since the migration of gravel is influenced by changes B in bed slope, the mass balance framework pre- y2 sented here may be used to evaluate changes in V2 subsidence over geological time. By locating the paleo–gravel front through future coring y1 V1 campaigns on the delta topset, this framework β ɑ could be used to reconstruct the associated tec- x2 x1 tonically induced subsidence rate by utilizing measurements of ancient gravel progradation Bifurcation Downstream Camp node limit of gravel distances and ancient channel slopes, which downstream (current bifurcation may be estimated from grain size. node) Conceptually, for a fixed location near the topset, the expected stratigraphy of the Selenga Figure 14. Perspective, three-dimensional view of a control volume Delta consists of several cycles of coarsening- used to compute the volume necessary to deposit sediment on a upward sediment deposits. The termination of ­Selenga River Delta distributary channel in order to increase chan- each cycle indicates a discrete earthquake event nel slope sufficiently to transport gravel downstream. See Table 3 for that produces subsidence of the delta topset. The values for each parameter, specified for each distributary channel.

14 Geological Society of America Bulletin, v. 1XX, no. XX/XX Geological Society of America Bulletin, published online on 9 March 2016 as doi:10.1130/B31427.1

Controls on gravel termination in seven distributaries of the Selenga Delta

TABLE 3. VARIABLES DEFINED IN FIGURE 14 FOR SEVEN DISTRIBUTARY CHANNELS

Measured average slope Slope needed x1 x2 y1 y2 B V1 V2 Vg Distributary channels (α) (β) (km) (km) (m) (m) (m) (km3) (km3) (km3) Lobanovskaya 0.000168 0.000336 23.0 9.23.9 10.8 173.60.0078 0.0300.023 Levoberezhnaya 0.000161 0.000303 16.7 7.02.7 7.2221.0 0.0049 0.0190.014 Galutay 0.000209 0.000355 22.4 5.94.7 10.1 109.80.0058 0.0160.0098 Kazanova 0.000159 0.000405 12.2 6.42.0 7.6103.7 0.0012 0.0073 0.0061 Kharauz 0.000155 0.000239 15.4 12.3 2.46.7 135.20.0024 0.0130.010 Sredneustie 0.000159 0.000470 14.5 13.3 2.313.140.00.000670.0072 0.0066 Kolpinnaya 0.000159 0.000527 14.8 9.82.4 13.0 27.2 0.000480.0044 0.0039 Total N.D.* N.D.* N.D.* N.D.* N.D.* N.D.* N.D.* 0.0230.100.073 Note: See Table 1 for average channel depth (H ). *N.D.—not determined.

several subsided delta topsets based on inter- material is trapped on the delta topset, and only Cui, Y., and Parker, G., 1998b, the Arrested gravel front: stable gravel-sand transitions in rivers part 2: general pretation of seismic images acquired adjacent fine material is transported into the basin trough. numerical solution: Journal of Hydraulic Research, to the modern delta. Drill cores penetrating the This latter supposition is confirmed by drill v. 36, p. 159–182, doi:​10​.1080​/00221689809498631​. topset and sampling several such cycles could cores and seismic data from Lake Baikal, which Delvaux, D., Fronhoffs, A., Hus, R., and Poort, J., 1999, Normal splays, relay ramps and transfer zones in unveil these coarsening-upward packages; find only fine sediment. the central part of the Baikal Rift basin: Insight from unfortunately, these data do not exist. Indeed, (5) The ultimate mechanism for the trapping digital topography and bathymetry: Bulletin du Centre­ de Recherches Elf Exploration Production, v. 22, these interpretations are bolstered by reviews of of gravel on the delta topset is subsidence due to p. 341–358. numerical work found in Paola (2000), whereby episodic tectonism. As of result of subsidence Dietrich, W.E., and Whiting, P., 1989, Boundary shear coarse-grain sediment fronts migrate basinward below the active channel depth (i.e., morpho- stress and sediment transport in river meanders of sand and gravel, in Ikeda, S., and Parker, G., eds., and retreat due to the influences of autogenic and dynamic reworking depth), the subsided topset River Meander­ing: American Geophysical Union allogenic cycles. For the Selenga Delta, allo- channel networks are preserved as part of the ­Water Resources Monograph 12, p. 1–50, doi:10​ ​.1029​ genic forcing appears to be the primary control, delta stratigraphy. /WM012p0001​. Frings, R.M., 2011, Sedimentary characteristics of the because the highly active tectonic situation sets This study provides data and analyses that gravel-sand transition in the River Rhine: Journal of the basinward displacement of the gravel front. contribute to evaluating sediment transport pro- Sedimentary Research, v. 81, p. 52–63, doi:​10.2110​ /jsr​ ​ .2011​.2​. cesses operating for shelf-edge deltas. The vari- Gao, S.S., Liu, K.H., Davis, P.M., Slack, P.D., Zorin, Y.A., CONCLUSIONS ation in the grain size of channel bed sediment, Mordvinova, V.V., and Kozhevnikov, V.M., 2003, Evi- i.e., gravel and sand transitions on a delta topset, dence for small-scale mantle convection in the upper mantle beneath the Baikal Rift zone: Journal of Geo- This study was devoted to linking the morpho­ as preserved in the stratigraphic record, could physical Research, v. 157, p. 871–889. dynamic nature of the Selenga River Delta top- provide insight into the changes in subsidence Gyninova, A.B., and Korsunov, V.M., 2003, The soil cover set, where a fluvial-dominated shelf-edge delta patterns over geological time. This in-depth of the Selenga Delta area in the Baikal region: Eurasian Soil Science, v. 3, p. 243–255. enters an active rift basin, to the stratigraphy study of the Selenga system could be applied Hack, J.T., 1957, Studies of Longitudinal Stream Profiles in of the system as influenced by tectonic activ- to other delta systems located in active basins, Virginia and Maryland: U.S. Geological Survey Pro- fessional Paper 294-B, 59 p. ity. Sediment transport processes are affected modern and ancient, to evaluate the influences Hutchinson, D.R., Golmshtok, A.J., Zonenshain, L.P., by spatially variable channel geometries, water of tectonics and climate on stratigraphy. Moore, T.C., Schloz, C.A., and Klitgord, K.D., 1992, discharge, and downstream reduction in shear Depositional and tectonic framework of the rift basins­ of Lake Baikal from multichannel seismic data: stress. These conditions influence the basinward ACKNOWLEDGMENTS Geol­ogy, v. 20, p. 589–592, doi:​10​.1130​/0091​-7613​ dispersal of sediment, in particular, gravel. The (1992)020​<0589:​DATFOT>2​.3​.CO;2​. findings of this study are as follows: We thank Vamsi Ganti and Robert Duller for pro- Il’icheva, E.A., 2008, Dynamics of the Selenga River network viding insights and reviews of this study. We thank and delta structure: Geography and Natural Resources, (1) The Selenga River Delta consists of eight v. 29, p. 343–347, doi:​10​.1016​/j​.gnr​.2008​.10​.011​. Rice University for the partial financial support. This orders of distributary channels. With increasing Il’icheva, E.A., Pavlov, M.V., Korytny, L.M., 2014, The river research was supported in part by National Science network of the Selenga Delta at present: Tomsk State channel order and accordance with downstream Foundation grant EAR-1415944. 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(98)00022​-4​. Printed in the USA

16 Geological Society of America Bulletin, v. 1XX, no. XX/XX Geological Society of America Bulletin, published online on 9 March 2016 as doi:10.1130/B31427.1

Geological Society of America Bulletin

Controls on gravel termination in seven distributary channels of the Selenga River Delta, Baikal Rift basin, Russia

Tian Y. Dong, Jeffrey A. Nittrouer, Elena Il'icheva, Maksim Pavlov, Brandon McElroy, Matthew J. Czapiga, Hongbo Ma and Gary Parker

Geological Society of America Bulletin published online 9 March 2016; doi: 10.1130/B31427.1

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