부산대학교 도서관

Earth surface movements are a primary external control on river system dynamics and evolution. It has often been observed that when responding to Earth surface motion driven by surface expression of folds, major rivers incise across young, active folds near their structural culminations and divert around others. This study shows that for the major rivers Karun and Dez in the Mesopotamian-Persian Gulf foreland basin, these different river responses are due to the need for narrow channel-belts to be maintained where a river incises across a fold, and the time it takes (at least several decades) for such narrow channel-belts to develop. In general, where a major river initially encounters a fold as an emerging fold “core”, the river flows across the uplifting fold for sufficient time for the development of a narrow channel-belt, thus producing an incising river course across the fold (a single “water gap”) in the vicinity of the fold “core” and the subsequent structural culmination. However, where a major river initially encounters a fold as a larger, emerged fold, the river does not flow across the uplifting fold for sufficient time, due to channel migration in response to lateral fold growth, thus producing a river course diverting around the fold “nose”. Hence, river reaches across the fold axis for river incision are characterised by narrow channel-belts, low channel sinuosities, high specific stream powers, and river crossing locations relatively near to the fold “core” (generally nearer than 16 km). By contrast, river reaches across the fold axis projection for river diversion are characterised by average channel-belt widths and channel sinuosities with fairly wide ranging values, fairly low specific stream powers, and river crossing locations relatively far from the fold “core” (further than 22 km). A narrow average channel-belt width of less than c. 2.7 km is a threshold for the rivers Karun and Dez (mean annual discharges c. 575 m³s¯¹ and 230 m³s¯¹) encountering folds in lowland south-west Iran (rates of uplift c. 0.1 - 2.3 mm yr¯¹), and this probably has a precedence over other geomorphological changes for producing river incision across a fold in response to uplift. In general, slightly smaller rivers are more frequently diverted around the fold “nose”, and small rivers and creeks, which are more easily “defeated” by fold growth, frequently develop a series of narrow “wind gaps” across a fold. The influences of human impacts on major rivers can be distinguished from those of Earth surface movements by suites of river characteristics. There may be significant interactions where these two external factors coincide, most notably where fold uplift and major anthropogenic river channel straightening produce the persistence of long, near-straight river courses (channel sinuosity < 1.1 over a river course > 10 km long).