Julie and I both included assessments of dam development on the Mekong in our dissertations, she on the Don Sahong and I on the Xayaburi. The Xayaburi Dam controversy I have written about previously several times. Xayaburi is the first mainstem dam in the Lower Mekong. There are at least five dams commissioned upstream in China, but those, for most people who work in the Mekong, are a non-issue. The Laos-Thai Xayaburi created a real buzz with the international community and continues to hold symbolic relevance to irreversible changes in the region. These changes are particularly important to fisheries, water chemistry, sediment load, subsistence communities, to name a few issues.
When I traveled to Laos to conduct research in 2013, I found that through the eyes of locals, the region had changed some years before - right about the time of the Chinese storage dam commissioning coupled with some economic changes in Lao domestic economic policy and the organization known as ASEAN. Though Xayaburi gained international attention, changes in the Mekong, remarkable changes to biodiversity, water flow, and land-use, were already causing some subsistence communities to migrate away from traditional lifestyles. Don Sahong will just continue the trend, now with more potential direct impacts on Cambodia's fisheries.
While Xayaburi broke a precedent of no big dam development on the Mekong River (sort of) Don Sahong is a real problem because it will be located where the last remaining Mekong Irrawaddy dolphins live, reproduce, and play as well as very close to the Cambodian border. When I was in Laos in 2013 you could rent a kayak and go see the animals - a rare experience. Julie talked to people about Sahong working in the region and the implications to people living on the river & people dependent on the fish downstream. (make sure to scroll down on the link - the first page is blank!)
Southeast Asia in general has seen great economic changes in the last two decades and continues to grow in population and economies. The water resources naturally come under threat in such circumstances, especially with all this international push for "green" energy - something that in reality hydropower is not.