Or HaChaim on Genesis 41:2/אור החיים על בראשית מ״א:ב׳

1 מן היאור עולות, from the river there were rising, etc. The Torah should have written: עולות שבע פרות מן היאור, "seven cows were rising from the river." Why was the river mentioned first? Perhaps the Torah wanted us to appreciate that the existence of those cows was entirely due to the river, as if the river itself had produced them. To this end the Torah first had to mention who it was that created these cows. What the river produced became of secondary importance. Had the Torah employed the normal syntax we would have thought that the cows crossed the river and rose on the far bank but not that they were a product of the river itself. This very fact also made Pharaoh aware that it could only be a dream. In real life rivers do not produce cows. The reason he was given such a vision was to alert Joseph to the fact that two separate products emanated from the river, i.e. the years of plenty and the years of famine. The river was the key to the ability of the cows to rise or not to rise.