‘ And Joshua said to the people, “Sanctify yourselves, for tomorrow YHWH will do wonders among you.” '
The uniqueness of the occasion is again stressed here. They were about to see the wonders of YHWH. They must therefore be ‘sanctified' in preparation for it. This would include washing their clothes thoroughly and abstaining from sexual contact (Exodus 19:10; Exodus 19:15; Genesis 35:2).
“Tomorrow YHWH will do wonders among you.” This was the reason for the special requirements. They were to behold the wondrous working of YHWH that previously they had only heard about. YHWH was about to come near and manifest Himself. Compare Exodus 4:13 where Moses also spoke of seeing the deliverance of YHWH. So they were to be sanctified ‘lest YHWH break forth on them' (Exodus 19:22).
The similarity between this and the crossing of the Sea of Reeds was to be recognised. They had left Egypt by passing though the waters, they would enter Canaan by passing through the waters. It was a new beginning, a new birth, brought about by the miraculous power of YHWH. In a sense it was the adoption of the new generation of Israel. We must not read cleansing into this passing through the waters for that idea is not prominent in the Old Testament. They spoke rather of the power and manifestation of YHWH - Psalms 114:3; Psalms 114:5; Psalms 114:7. In the Old Testament water spoke of new life and deliverance (Isaiah 44:3-5; Isaiah 32:15). You could always tell where there was water, because there there was fruitfulness and life. (Washing with water in the ritual was always preparatory to cleansing, not a vehicle of cleansing in itself unless it was sprinkled with the ashes of a heifer and thus became ‘cleansed water').
This revealing of wonders was deliberate on the part of YHWH. At this time of the year (April) the Jordan overflowed its banks. At any other time the fords could have been used to cross it, but not at this time. The Israelites were to cross the Jordan when the river was at its widest and deepest, and was flowing its swiftest. They were not, of course, aware of this. They were not familiar with the Jordan. As the snow on Mount Hermon melts and the rainy season ends, the Jordan rises at this season to a depth of 10-12 feet (3-4 metres) and floods to a width of 300-360 feet (100 metres) at this point, unlike the slow moving, turgid river of the dry season.
But in this crossing of the Jordan there was a divine necessity. It could not wait. It was to be the springboard to which they would look back and remember that YHWH was with them. Whenever they doubted they would remember the crossing of the Jordan, and how YHWH had therefore put the land at their disposal. And it was to be the complete validation before the people that Joshua was the new Moses (verse 7).