Three-dimensional unmanned aerial vehicle (UAV) oblique photogrammetric data were used to infer mountainous gravel braided river lithofacies, lithofacies associations and architectural elements. Hierarchical architecture and lithofacies associations with detailed lithofacies characterizations were comprehensively described to document the architectural model, architectural element scale and gravel particle scale. (1) Nine lithofacies (i.e., Gmm, Gcm, Gcc, Gci, Gcl, Ss, Sm, Fsm and Fl) were identified and classified as gravel, sand and fine matrix deposits. These are typical depositional features of a mountainous dryland gravel-braided river. (2) Three architectural elements were identified, including channel (CH), gravel bar (GB) and overbank (OB). CH can be further divided into flow channel and abandoned channel, while GB consists of Central Gravel bar (CGB) and Margin Gravel bar (MGB). (3) The gravel bar is the key architectural element of the gravel braided river, with its geological attributes. The dimensions of GBs and their particles are various, but exhibit good relationships with each other. The grain size of GB decreases downstream, but the dimensions of GB do not. The bank erosion affects the GB dimensions, whereas channel incision and water flow velocity influence the grain size of GB. The conclusions can be applied to the dryland gravel braided river studies in tectonically active areas.