Genre and place in the book. One of the most famous images of the Meditations: time as a river of becoming (ποταμὸς τῶν γινομένων) and a violent stream (ῥεῦμα βίαιον). The Heraclitean flux applied to time; it continues the book's "mutability" thread (04-36, 04-42).
The river and the stream. αἰών here is not "eternity" but time-as-age, the continuous flow of becoming. The river's waters are τὰ γινόμενα, "the things coming to be." The image conveys instantaneity: ἅμα… ὤφθη — "as soon as it is seen" — καὶ παρενήνεκται — "it has already been carried past"; another rushes into its place, and it too will be borne away. Nothing abides (cf. Med. 6.15 — "in this flowing stream, where nothing is stable, what is there to prize?").
The point. The transience of everything: since nothing abides, it is pointless to cling to the passing or to prize it. Only the present is real — and that but for a moment; and it alone can be lost (the present; cf. Med. 2.14 — one cannot lose the past or the future, only the current "now"). So the image works against attachment and against the fear of loss: both what is gained and what is lost are only water that has flowed by.
The Heraclitean background. "All things flow" (πάντα ῥεῖ — the later summary of the doctrine); the genuine river-fragment: ever-different waters flow over those who step into the same rivers (Heraclitus, DK B12; the "cannot step in twice" form is the paraphrase B91 / Plato). Marcus appeals to Heraclitus directly a little later (Med. 4.46) and constantly thinks of the world as a stream.
The disciplines. The principal one is desire: the contemplation of the stream reconciles us to the flow and weans us from clinging to the passing. The secondary is assent: the right judgement of reality — all flows, nothing stands.
The exercise. EXERCISEContemplation of nature: the flux-image is "physics as therapy," a way of seeing that holds all things in the flow of becoming (the flux-facet, beside the facets of change in 04-36 and of unity in 04-40).
Parallels. The stream of becoming, nothing abiding — Med. 5.23; 6.15 (cf. EXERCISEcontemplation-of-nature). The Heraclitean flux — DK B12 (cf. heraclitus). Only the present is real and losable — 04-35; Med. 2.14 (cf. paron).