Water diversion from rivers to lakes is popularly considered as a measure to control lake eutrophication, but the actual efficacy is quite limited in general. How to connect lakes with rivers in a scientific way? To answer this question, first, I analyze 5 diversion cases in China and other countries, and simulate effect of adding water using Vollenweider’s model. The results show that improving water quality by dilution depends upon whether there is large amount of input water with total phosphorus concentrations much lower than original inflow ones, but such a premise is generally difficult to meet. Second, I compare water quality of lakes disconnected and connected with the Yangtze mainstem, and find that chlorophyl a
concentration of water in disconnected lakes are increased by 0.7—2.4 times when total phosphorus concentrations are in the same levels. The mechanisms of better water quality of connected lakes are fourfold, i.e., the natural water level fluctuation promotes vegetation development, seasonal trying-up and shallowing of lakeshore promotes pollutants decomposition and adsorption, turbid flowing water results in lower efficiency of nutrient utilization by phytoplankton, and the ecological complexity is higher with more heterogeneous habitats, higher biodiversity, and more complex food webs. Last, I point out that setting a scientific feasible target is the very basis to implement a river-lake connection project correctly. If the aim is to dilute pollutants, enough clean water is needed, and this can only be realized for smaller waterbodies. If the aim is to wash out phytoplankton, the flushing rate should be higher than specific growth rates of dominated algae, reaching 10%—20%/d, and this is also only feasible for smaller waterbodies. As for most lakes, in particular, shallow ones, connection of lakes with rivers should aim mainly to rehabilitate free hydrological connectivity and natural water level regimes, and then to enhance self-purification function. Such a mode costs a lesser amount of water, but reaches the best effect. This review provides important guidance for river-lake connection projects.