Channel evolution under changing hydrological regimes in anabranching reaches downstream of the Three Gorges Dam

Elucidating the influence of dams on fluvial processes can benefit river protection and basin management. Based on hydrological and topographical data, we analyzed channel evolution in anabranching reaches under changing hydrological regimes influenced by the Three Gorges Dam. The main conclusions are as follows： 1） the channels of specific anabranching reaches were defined as flood trend channels or low-flow trend channels according to the distribution of their flow characteristics. The anabranching reaches were classified as T1 or T2. The former is characterized by the correspondence between the flood trend and branch channels, and the latter is characterized by the correspondence between the flood trend and main channels; 2） on the basis of the new classification, the discrepant patterns of channel evolution seen in anabranching reaches were unified into a pattem that showed flood trend channels shrinking and low-flow trend channels expanding; 3） flood abatement and the increased duration of moderate flow discharges are the main factors that affect channel adjustments in anabranching reaches after dam construction; and 4） in the next few decades, the pattern of channel evolution will remain the same as that of the Three Gorges Dam operation. That is, the morphology will fully adapt to a flow with a low coefficient of variation. Our results are of interest in the management of the Yangtze River and other rivers influenced by dams.

Classification of River Reaches on the Little Disturbed East Alligator River, Northern Australia

The East Alligator River drains a 7000 km2 catchment in northern Australia comprised largely of Aboriginal land and has a very low average population density of about 0.15 persons per km2. River reaches were classified according to geomorphic features on both the East Alligator River and its major tributary, Magela Creek, next to which is located the Ranger uranium mine. Sixteen reaches were described for the 241.4 km of the East Alligator River and ten reaches were described for the 118.8 km of Magela Creek. The dominant river types on the East Alligator River were various types of anabranching rivers, sandstone gorges and cuspate tidal meanders. On Magela Creek the dominant river types were wetlands and channel billabongs, island anabranching and sandstone gorges. It is unusual for anabranching rivers and gorges to be so dominant. Current river classification schemes could not accommodate all the various river types and need to be modified so that tidal channels are covered in more detail and so that the classification of anabranching rivers recognises that sand-bed varieties occur in partly confined valley settings.

ANABRANCHED RIVERS

Unlike other river patterns, the anabranched pattern is rarely studied before although Lane has already mentioned about it in 1957. The most eminent features of the anabranched pattern are that there are always some islands developed in the alluvial rivers and thus several stable flow courses always exist correspondingly.This is quite different from the unstable islands and randomly changed flow courses in braided rivers. Since the anabranched river pattern is the most typical one in the middle and lower reaches of the Yangtze River (Changjiang) as well as in many other rivers in China, a systematic study on the formation cause of the anabranched river pattern has been made in recent years, including field observations, modeling experiments in four river models under different boundary conditions, the analyses of river pattern formation and transformation etc. In the present paper, some of the detailed results obained by the writer will be discussed.

Geodiversity in the Yellow River source zone

It is a key premise of ＇ecosystem approaches＇ to natural resources management that we must have an appropriate understanding of biodiversity values, and controls upon them, if we wish to manage them effectively. These biodiversity values, and associated ecosystem functionality, vary with space and time and are tied directly to landscape-scale relationships and evolutionary traits. In riverine systems, nested hierarchical principles provide a useful platform to assess relationships between landscape components across a range of scales. These understandings are most instructively synthesized through catchment-scale analyses. This paper outlines a rationale for systematic catchment-wide appraisals of river geodiversity. An initial application of these principles is presented for the Yellow River source zone in Qinghai Province, western China. Geo-ecological relationships are outlined for five broad sections of the trunk stream, highlighting implications for the management of these individual landscape compartments and for the system as a whole.