Planform characteristics and development of inter- channel wetlands in a gravel-bed anastomosing river, Maqu Reach of the Upper Yellow River

Both interchannel wetlands and multi-channels are crucial geomorphologic units in an anastomosing river system. Planform characteristics and development of interchannel wetlands and multi-channels control the characteristics of anastomosing rivers. To under- stand the role that interchannel wetlands play in the development of anastomosing rivers, a study was conducted on the Maqu Reach of the Upper Yellow River （MRUYR）, a gravel-bed anastomosing river characterized by highly developed interchannel wetlands and ana- branches. Geomorphologic units in the studied reach were extracted from high resolution satellite imagery in Google Earth. The size distributions of interchannel wetlands and inter- channel wetland clusters （IWCs）, a special combination of interchannel wetlands and ana- branches, were investigated. Geomorphologic parameters, including the ratio of interchannel wetland area to IWC area （P）, shoreline density （DL）, and node density （Dn） were used to analyze planform characteristics of IWCs and the development of multi-channels in the stud- ied reach. The results suggest that small or middle sized interchannel wetlands and large or mega sized IWCs are more common at the study site. The area of IWC （Su） is highly cor- related with other geomorphologic parameters. P increases with increasing Su, and the upper limit is about 80%, which indicates that the development of interchannel wetlands and ana- branches in the IWC is in the equilibrium stage. In contrast, D~ and D, show a tendency to decrease with increasing Su due to diverse evolution processes in IWCs with different sizes. There are three main reasons leading to the formation of IWCs： varying stream power due to the meandering principal channel; development of the river corridor due to the weakening of geologic structure control; and high stability of interchannel wetlands due to conservation by shoreline vegetation.

Wandering gravel-bed rivers and high-constructive stable channel sandy fluvial systems in the Ross River area,Yukon Territory,Canada

Mid-Cretaceous strata within the Tintina Trench, 3 km west of the community of Ross River, contain evidence of deposition in two distinct, alternating, fluvial settings. Coal-bearing, mud-dominated strata are commonly associated with high-constructive sandy channel systems, with extensive overbank, levee and splay deposits. Channels are between 3 and 30 m wide and 0.4-7 m thick. They show repetitive development of side and in-channel bar-forms, as well as up-channel widening of the rivers by selective erosion of associated overbank and levee deposits. Levees extended for several hundred metres away from the channels. In this setting low-angle inclined stratification and epsilon cross stratification may reflect lateral migration of crevasse channels or small streams. The paucity of exposure prevents recognition of the channels as products of multiple channel anastomosed systems or single channel high-constructive systems. Gravel-dominated strata, inter-bedded with, and overlying coal-bearing units, are interpreted as deposits of wandering gravel-bed rivers, with sinuosity approaching 1.4. In most exposures they appear to be dominated by massive and thin planar-bedded granule to small pebble conglomerates, which would traditionally be interpreted as sheet-flood or longitudinal bar deposits of a high-gradient braided stream or alluvial fan. Architectural analysis of exposures in an open-pit shows that the predominance of flat bedding is an artefact of the geometry of the roadside exposures. In the pit the conglomerates are dominated by large scale cross stratification on a scale of 1-5.5 m. These appear to have developed as downstream and lateral accretion elements on side-bars and on in-channel bars in water depths of 2-12 m. Stacking of strata on domed 3rd order surfaces suggests development of longitudinal in-channel bar complexes similar to those observed in parts of the modern Rhone River system. Mudstone preserved in some of the channels reflects intervals of channel abandonment or avulsion. Minimum channel width is from 70 to 450 m.

Analysis of controls upon channel planform at the First Great Bend of the Upper Yellow River, Qinghai-Tibet Plateau

The 270 km long section of the Upper Yellow River at the First Great Bend is comprised of single channel and multiple channel systems that alternate among anastomosing, anabranching, meandering and braided reaches. The sequence of downstream pattern changes is characterized as： anastomosing-anabranching, anabranching-meandering, meandering-braided and braided-meandering. Remote sensing images, DEM data and field investigations are used to assess ahd interpret controls on these reach transitions. Channel slope and bed sediment size are key determinants of transitions in channel planform. Anas- tomosing reaches have a relatively high bed slope （0.86‰） and coarser sediment bed material （d50 = 3.5 mm）. In contrast, meandering reaches have a low slope （0.30‰） and fine sediment bed material （d50 = 0.036 mm）. The transition from a meandering to braided pattern is characterized by an increase in channel width-depth ratio, indicating the important role of bank strength （i.e. cohesive versus non-cohesive versus channel boundaries）. Interestingly, the braided-meandering and meandering-braided transitions are coincident with variable flow inputs from tributary rivers （Baihe and Heihe rivers respectively）. Theoretical analysis of the meandering-braided transition highlights the key control of channel width-depth ratio as a determinant of channel planform.