Functioning freshwater ecosystems are widely recognized as a planetary boundary for the continued human inhabitation of our planet, but little is known about the tradeoffs at the nexus of food, energy and water. In this dissertation I explored the effects of hydrologic variability in the Lower Mekong Basin (LMB) on rice production and functional structure of fish catches. I then examined the tradeoffs at the intersection of fish and rice harvest as a function of hydrologic variability and modeled production under novel engineered hydrologic scenarios. I modeled rice production using a Multivariate Autoregressive State Space (MARSS) model and mechanistically tested for the effect of saline intrusion. I found rice production to be heterogeneously affected by hydrology; in saline afflicted areas, floods had a positive effect size on production, whereas in non-saline afflicted areas, floods had a negative effect size on production. To address hydrologic filtering of the functional structure of fish catches, I collected thousands of specimens from over 100 LMB species in collaboration with Cambodia’s Inland Fisheries Research and Development institute and the Royal University of Agriculture. LMB fishes comprise a large portion of the 1,200 known species in the basin and have historically provided a substantial amount of animal protein to 60 million people in the region. Using an RLQ, co-inertia analysis, I found four functionally relevant morphological trats that were significantly associated with hydrologic variation—mouth position, maxillary length, relative body depth, and relative head depth. These traits are associated with many of the threated species in the LMB, which make up a large portion of the 1200 known species in the basin and have historically provided a substantial amount of animal protein to 60 million people in the region. To examine the tradeoffs within food systems, I used MARSS maximum likelihood estimation to forecast fish and rice production throughout the LMB under different hydrologic scenarios. I end my dissertation with an opinion piece on NexGen Mekong Scientists, a program I started in 2020 with funding from the United States Department of State.