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

Abstract

The Selenga River Delta, Lake Baikal, Russia, is ~600 km 2 in size and contains multiple distributary channels that receive varying amounts of water and sediment discharge. The delta is positioned along the deep-water (~1600 m) margin of Lake Baikal, a half-graben-styled rift basin, qualifying it as a modern analogue of a shelf-edge delta system. This study provides a detailed field survey of channel bed sediment composition, channel geometry, and water discharge. The data and analyses presented here indicate that the Selenga Delta ex hibits downstream sediment fining over tens of kilometers, ranging from predominantly gravel (coarse pebble) and sand near its apex to silt and sand at the delta-lake interface. We developed an analytical framework to evaluate the downstream elimination of gravel within the multiple distributary channels. The findings include the following. (1) The Selenga River Delta consists of at least eight orders of distributary channels. (2) With increasing channel order downstream, channel cross-sectional area, width-depth ratio, water discharge, boundary shear stress, and sediment flux systematically decrease. (3) The downstream elimination of gravel in distributary channels is caused by declining boundary shear stress as a result of water discharge partitioning among the bifurcating channels. (4) Over longer time scales, gravel is contained on the delta topset due to frequent and discrete seismic events that produce subsidence and accommodation, so that coarse sediment cannot be transported to the axis of the Baikal Rift basin. The distribution of sediment grain size in deltaic channels, as related to hydrodynamics and sediment transport, plays a critical role in influencing stratigraphy, because the sustained tectonism leads to high preservation potential of the delta topset sedimentary deposits. Therefore, the Selenga River Delta provides an opportunity to explore the interactions between modern deltaic sedimentation processes and tectonics that affect the production of basin stratigraphy.

Roles of Bank Material in Setting Bankfull Hydraulic Geometry as Informed by the Selenga River Delta, Russia

“…Shields number decreases with increasing , and increases with increasing slope in each grain‐size class (Figures a and b), these patterns are consistent with results from previous studies (Dade & Friend, ; Li et al, ; Parker et al, ; Trampush et al, ; Wilkerson & Parker, ). Furthermore, bankfull Shields number varies by nearly an order of magnitude within ∼35 km of channel network of the Selenga Delta (Figure e), because bed sediment size, slope, and bankfull depth all decrease downstream due to bifurcation (Figures b and a–d) (Dong et al, ).…”

“… (a) Calculated bankfull water discharge versus channel order (modified from Dong et al, ). (b) Slope versus channel order.…”

“…nearly an order of magnitude within $35 km of channel network of the Selenga Delta (Figure 8e), because bed sediment size, slope, and bankfull depth all decrease downstream due to bifurcation (Figures 3b and 8a-8d) (Dong et al, 2016).…”

Sedimentation patterns in the Selenga River delta under changing hydroclimatic conditions

“…The delta is characterized by eight orders of natural distributary channels that partition water and sediment over the course of approximately 35 km. A recent study has documented that water partitioning among the channel orders produces a reduction in stream power and an associated non‐linear reduction in sediment transport (Dong et al, ). This, in turn, produces deposition of sediment within the channel network system, with the coarsest sediment load (gravel) eventually giving way to sand and silt, starting at the delta's apex and progressing downstream to the termination of distributary channels (Dong et al, ; Ilyicheva, Gagarinova, & Pavlov, ).…”

“…The Selenga River delta is a fluvially dominated fresh‐water system that is characterized by up to eight orders of distributary channels; as such, the delta region has developed large lakes and widespread wetland regions that are adjacent to the channels (Lane, Anenkhonov, Liu, Autrey, & Chepinoga, ; Dong et al, ; Chalov, Thorslund, et al, ). The delta stores sediment (up to 40% of suspended and 70% of the total sediment load during high discharge conditions ~3,000 m 3 s −1 ) and particle‐bound metals originating from upstream extensive mining areas through dispersal and deposition of fine sediment where sediment‐laden water evacuates distributary channels and enters adjacent low‐energy wetlands (Chalov, Thorslund, et al, ).…”