10.2478/v10060-008-0067-5 Annals of Warsaw University of Life Sciences – SGGW Land Reclamation No 42 (1), 2010: 79–91 (Ann. Warsaw Univ. of Life Sci. – SGGW, Land Reclam. 42 (1), 2010) Stream networks and knickpoints in the Sanjiangyuan Region ZHAOYIN WANG1, GUO-AN YU2, GARY BRIERLEY3, LE LIU4 1State Key Laboratory of Hydroscience and Engineering, Tsinghua University, China 2Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences 3School of Environment, University of Auckland 4Department of Hydraulic Engineering, Tsinghua University, China Abstract: Stream networks and knickpoints in INTRODUCTION the Sanjiangyuan Region. The source area of the Lancang (Mekong), Yangtze and Yellow rivers is The 6,300 km Yangtze River and 5,464 km named in Chinese Sanjiangyuan (source of three Yellow River are the longest and second rivers). Geographical characteristics of these riv- ers, and various rivers at the margin of the Qing- longest rivers in China. The Lancang hai-Tibetan Plateau (Jialing, Minjiang, Dadu, River (Mekong River) is the largest Yalong and Jinsha) are summarized from fi eld international river in Asia. The source investigations along with digital elevation model area of the three rivers is named in (DEM) analyses and satellite images. Uplift of Chinese Sanjiangyuan (source of three the Qinghai-Tibetan Plateau has resulted in an rivers). It extends on the Qinghai-Tibet asymmetrical distribution of tributaries for stream o o networks at the margins of the plateau. Almost all plateau from N31 39’ to N36 16’ and o o tributaries join the trunk stream from the north- from E89 24’ to E102 23’. Altitude varies west. Three types of drainage network are evi- from 2,000-5,000 m, with a mean altitude dent: plume, nervation and dendritic. In general, of about 4,000 m. The total area of San- plume networks have a large number of fi rst order jiangyuan is 360,000 km2. About 49% streams. Nervation networks have a main stream of Yellow River water, 15% of Lancang and parallel tributaries. Dentritic networks are characrterized by continuous bifurcation and have River water and 2.5% of the Yangtze a branch-like appearance. Most stream networks River water come from this region in the Sanjiangyuan region are of the nervation (http://www.snowland-great-rivers. type. Several large knickpoints are evident along org/threerivers.htm). There are more the longitudinal profi les of the Yellow and Yang- than 180 streams, 16,500 lakes, a total tze Rivers. Channel aggradation immediately up- of 73,300 km2 of swamp, and numerous stream of these knickpoints marks a transition in river processes from vertical bed evolution (i.e. snow mountains and glaciers with a total 2 incision) to horizontal channel adjustment (and area of 2,400 km in the region. associated braided and anabranching channels). Northward movement of the Indian Plate, and its collision with the Eurasian Key words: Qinghai-Tibetan Plateau, knickpoint, Plate, has uplifted the Qinghai-Tibetan drainage network, Sanjiangyuan, step-pool. Plateau, which is referred to as the ‘third 80 Zhaoyin Wang et al. pole’ or the ‘roof of the world’. Uplift of ciated high sediment transport rates, has the Qinghai-Tibetan Plateau has created resulted in various ecological problems drier and colder conditions (Tang and in the Sanjiangyuan region. Dong 1997). It has also brought about This paper overviews summary results large-scale and profound environmental from fi eld and DEM-based analyses on changes in surrounding areas, including stream network patterns in the Sanjiangy- the formation of deserts in northwestern uan region and knickpoint processes at China (Dong et al. 1994). the margin of the Qinghai-Tibetan Pla- Various stream network patterns have teau. Results from these scientifi c inves- formed under different environmental tigations will hopefully provide effective conditions in the region. Rather than guidance for integrated management of developing a typical dendritic (tree-like) natural resources in the Sanjiangyuan form, stream networks have a nervation region, aiding sustainable development (vein-like) form (Wang et al. 2009b). programmes that strive to reconcile eco- Hortonian laws of stream numbers, logical protection with sustainable use of lengths and drainage areas vary mark- land and water resources. edly across the region, with notable dif- ferences in glaciated and non-glaciated METHODS areas. Indeed, glacial and fl uvial erosion are the main driving forces in the deve- Geographical and geometrical characte- lopment of complex stream-lake network ristics were measured in the fi eld using in the region. Global Positioning System (GPS) and Deformation of landscapes at the pla- laser range meter. A 1995 digital map teau margin has resulted in steep, highly of China with a scale ratio of 1:250.000, dynamic rivers, many of which are cha- 1960s topographic maps with a scale ratio of 1:50 000, and 2004 digital topographic racterized by typical step-pool system SRTM-3 data were used to construct sequences (Chin 2002; Costa and Schus- longitudinal profi les and cross sections ter 1988; Korup 2004; Wang et al. 2009a). of various rivers. This was completed An approximate topographic equilib- using ARCGIS. GoogleEarth was used to rium has been maintained as rapid fl u- provide global satellite images at varying vial incision into bedrock and headward resolutions. In this study, the “eye alti- migration of knickpoints strives to keep tude” was fi xed to ensure that all data were balance with tectonic uplift (Korup et al. collected at the same resolution. 2006; 2009). This has further accentua- ted slope instability, so landslides and debris fl ows are common (Ouimet et al. ASYMMETRICAL DISTRIBUTION 2007). High sediment loads carried by OF TRIBUTARIES the Yellow, Yangtze and Lancang Rivers According to the British Geological refl ect high rates of landscape denuda- Survey, the Indian Plate moves northward tion. This has increased sediment trans- at a rate of 5 cm/year (Chen and Gavin, port by rivers and offshore deposition. 2008). Its collision with the Eurasian Plate Landscape instability brought about by has resulted in the uplift of the Himalaya incision and mass movements, and asso- and the Qinghai-Tibetan Plateau, and Stream networks and knickpoints... 81 associated earthquake activity. The rate Jialing) are all tributaries of the Yangtze of horizontal movement is about 5 cm/yr River. The rising plateau has increased at Himalaya, 4 cm/yr in the source area the gradient of these rivers, inducing of the Lancang and Yangtze Rivers, to incision that often extends over 2 km about 2 cm/yr at the Qilian Mountain. deep. Uplift has caused dramatic chan- As a result the Himalaya rises at a rate ges to stream networks. All major rivers of 21 mm/yr while the Qilian Mountains in this region fl ow from northwest to rise at a rate of 5 mm/yr. The Sichuan southeast, refl ecting the uplift of the pla- basin, located to the east of the plateau, teau and relatively stationary position of is stationary. This pronounced variability the Sichuan Basin to the east. in the rates of horizontal movement has Asymmetry of stream networks is also resulted in many active faults, such as evident in small watersheds. Figure 2 the Kalakunlun Fault to the west, Kunlun shows the stream network of the Xihan- fault in the center, Altyn fault to the north, shui River (the upstream area of the Jial- and Xianshuihe and Longmenshan faults ing River) on the northeastern margin of to the east. The Wenchuan earthquake the plateau. Once more, all large tribu- on May 12, 2008 occurred along the taries join the river from the northwe- Longmenshan fault. stern side, creating a very asymmetrical Streams along the east margin of the stream network. More than 90% of the Qinghai-Tibetan Plateau and the Long- drainage area is on the western side of menshan Fault are shown on Figure 1. the river. This situation is repeated for The four major rivers of Sichuan Pro- many stream networks at the margins of vince (Minjiang, Tuojiang, Fujiang and the Qinghai-Tibetan Plateau. FIGURE 1. Streams on the east margin of the Qinghai-Tibetan Plateau and the Longmenshan Fault 82 Zhaoyin Wang et al. FIGURE 2. Asymmetrical distribution of tributaries and drainage area of the Xihanshui River FIGURE 3. Wenchuan earthquake changed the bed slope and cut off the fl ow in a tributary stream of the Shenxi Ravine in the Minjiang River basin Shenxi Ravine, a tributary stream in east (Fig. 3). The most recent uplift phase the Minjiang River basin, is located along of the plateau, the Wenchuan earthquake, the Longmenshan Fault which separates induced a 4 m vertical displacement along the Qinghai-Tibetan Plateau to the north- this fault, as the area to the northwest was west and the Sichuan basin to the south- uplifted. It triggered numerous landslides Stream networks and knickpoints... 83 and avalanches (Wang et al. 2009a). orders). However, various researchers Prior to the earthquake, the stream shown have questioned the universality of Hor- in Figure 3 was a perennial stream that ton’s law. For instance, Kirchner (1993) fl owed into the Shenxi Ravine. Tectonic argued that Horton’s ratio may vary for motion cut off surface runoff and changed different stream networks. Also, Liu and the slope of the stream. Such episodes Wang (2008) found that Horton’s ratio have been repeated many times during the is constant only for the stream orders millions of years over which uplift of the higher than 8. For lower stream orders, Qinghai-Tibetan Plateau has taken place. Horton’s ratio varies with stream order This has resulted in very asymmetrical and is very different for different river stream networks. networks. For example, a large number of fi rst order streams fl ow into a second STREAM NETWORKS ON THE order stream in a plume network (Fig.