Mega dam‐induced riverbed erosion exacerbates drought effects on river water surface elevation

Abstract

AbstractBy regulating the seasonal flow discharge, dams are frequently used to augment downstream flow discharges and water surface elevations in dry season periods. However, dams also typically generate severe bed incision in their downstream reaches, which whilst not reducing the volume of flow passing through the channel, can nevertheless reduce water surface elevations. Such reductions in water surface elevations can make it more difficult to access available water in the river in instances where they drop substantially below the levels of water intake structures. Here, we evaluate the extent to which dam‐induced incision downstream of the Three Gorges Dam (TGD) contributes to falling water surface elevations during the 2019 dry season (December to February), which was a period characterized by extremely low water surface elevations along the mid‐lower reaches of the Changjiang River. Our results indicate that the 2019 dry season exhibited the second lowest water surface elevation ever recorded in the middle Changjiang even though the flow discharge was actually larger than in previous dry seasons. The 2019 event was found to be characterized by a sharp fall of water surface elevation, caused by TGD‐induced downstream channel narrowing and bed incision. Thus, whilst TGD releases did augment flows during the 2019 dry season, channel degradation resulted in a substantial net lowering of water surface elevations. Consequently, the overall impact of the TGD on dry season water surface elevations was to aggravate (not mitigate) low water surface elevations experienced along the Changjiang River. Our study has relevance for other major dam‐altered rivers that experience extreme low water surface elevations in dry periods.