Mitigating Migratory Fish Impacts from Upper Basin Hydro-Dam Development on the Mainstem Mekong River Using Tourist Photographs

The Mekong River ecosystem is on the brink of unprecedented habitat loss and migratory fish exclusion. Mainstem Hou Sahong Dam, at the Laotian/Cambodian border, will be a critical blow to dry and wet season fish migration past the Great Fault Line. Pakse stream gage, 100 km upstream, has been recording mainstem Mekong riverflow since 1923. One looming ecological impact of accelerated mainstem dam/reservoir development above Pakse since early-2000 is an altered annual hydrograph. Mainstem hydro-dam reservoirs store peak runoff events to generate steadier, lower magnitude flow releases through the turbines, thus suppressing peak releases downstream. Our objectives are: (1) show how peak riverflows create wet season fish passage opportunities then (2) propose an effective engineering strategy mitigating lost wet season passage opportunities.

A steep cascade (a series of bedrock steps) within the Great Fault Line, Tat Somphamit, is at the far eastern boundary of the Liphi Waterfall complex. As floodwaters rise, river stage sharply increases at the cascade's base from a prominent backwater originating in the constricted canyon downstream. This annual backwater can drown-out all 8 bedrock steps depending on storm magnitude and duration. Before Tat Somphamit Cascade passes fish freely, all bedrock steps must be backwatered. Utilizing dated tourist photographs posted on the internet, we could assign an exceedence probability for step backwatering to each photograph using Pakse gage's pre-dam (1923 to 1980) daily average flow duration curve. A 32,000 cms riverflow with a background exceedence of 5.36% backwaters all but the two short bedrock steps at the cascade's crest. And both might be partial migration barriers (i.e., limited migration by stronger fish species). The post-dam flow duration curve (since 2003) assigned a 3.58% exceedence to 32,000 cms. Future peak flow suppression by 5% will reduce migratory opportunities up to 40% measured as the annual number of days riverflow exceeds 32,000 cms. Rather than installing an extensive ladder from the cascade's top to bottom approximately 30 m high, only approximately two 1.5 to 2.0 m high structures near the cascade's crest would significantly mitigate wet season fish passage. Tat Somphamit Cascade is one seasonal passageway among potentially hundreds.