A floodgate effect is situation in which a small action can result in a far greater effect with no easily discernible limit. The original analogy

Analogy

Analogy is a cognitive process of transferring information or meaning from a particular subject to another particular subject , and a linguistic expression corresponding to such a process...

 is that of a floodgate

Floodgate

Floodgates are adjustable gates used to control water flow in flood barriers, reservoir, river, stream, or levee systems. They may be designed to set spillway crest heights in dams, to adjust flow rates in sluices and canals, or they may be designed to stop water flow entirely as part of a levee or...

, which once opened, no matter how minutely, will allow water to flow from either side through the gate until both sides are balanced up. It may also be used to refer to the effect where, once a floodgate has been opened, water will gush out in a torrent through the gate, making it easier to continue to open the gate, but harder to close it.

The term is commonly used to illustrate a situation where a precedent will set the stage for repeated performances, the number of which is hard to control. Such an example can be illustrated as follows:

The setting up of a redlight district here will create a floodgate effect, causing redlight districts to be set up elsewhere.