A miraculous stream flowing from the Temple reaches its ultimate peak in its fourth stage of expansion. After an additional thousand cubits are measured, the waters transform into a mighty, impassable river. The primary approach among commentators is that the water deepens and rises to such an extent that regular walking is no longer possible. The current becomes so powerful that anyone attempting to cross on foot risks being swept away. Instead, the river turns into waters meant only for swimming and floating [רש"י, רד"ק, מצודת דוד, מנחת שי, ביאור שטיינזלץ].
Beyond this physical reality, the rushing river serves as a profound symbol for the future spread of wisdom, Torah, and faith throughout the world. The water emerges from the right side of the altar, representing kindness, goodwill, and gratitude, rather than from the north side, which symbolizes atonement for sins and the fear of punishment. The four distinct measurements of the river correspond to four levels of spiritual awareness humanity will achieve in the future. While the other nations of the world will attain partial spiritual understanding, the nation that received the Torah will reach the ultimate level. This highest tier is represented by the deep, swimming waters, symbolizing an overflowing spring of wisdom and a boundless knowledge of God [מלבי"ם].
Expanding on this symbolism, the flow of the river also mirrors the structure of the universe. In the higher spiritual realms, the water begins as a mere trickle, reflecting the limited capacity of humans to grasp those hidden domains. However, as the water descends into our physical reality, it swells into a massive river. This represents the vast, deep ocean of natural, scientific, and astronomical knowledge available in the material world, which humans are able to actively explore and understand [מלבי"ם].
From another perspective, the river's expansion directly parallels the physical and spiritual layout of the Temple itself. The fourth and final stage of the river, which cannot be crossed on foot, corresponds to the Holy of Holies. Just as the deep, surging waters block ordinary human passage, the Holy of Holies remains completely sealed off to the public, accessible only to the High Priest on the Day of Atonement [אברבנאל].