Well of course the start of it predated the mass adoption of steam engines, but as I understand it the start is generally considered to be when steam engines were first put to use pumping water out of coal mines, that water then being used to flood canals to transport the coal. That synergy was incredibly powerful, making cheap coal available in cities which allowed urban populations to rapidly grow, providing a workforce for the factories which would eventually (not initially) also be coal powered. That's the industrial revolution as I learned it.