The present disclosure relates to delivery and release systems, such as core-shell particles. An exemplary composition according to the disclosure can include a degradable polymeric shell surrounding a core that includes a crosslinker, which can encompass a metal, such as chromium. The core-shell particles can be provided with a gel-forming polymer, such as a polyacrylamide, into a subterranean reservoir having conditions such that the shell of the core-shell polymer degrades, and the so-released metal is effective to at least partially crosslink the gel-forming polymer to form a gel. The so-formed gel can be effective to control water flow through the subterranean reservoir, such as in relation to a waterflood of the reservoir.