After the devastating storm of the flood, a slow process of restoration begins to return the world to its proper state. The fierce waters that had covered the earth start a gradual retreat back to their original sources. They are drawn back into the oceans, descend into the deep voids beneath the earth, and return to the sky through evaporation [אור החיים, רש״ר הירש, קאסוטו, ביאור יש״ר, יהל אור]. This retreat is a constant, continuous movement from the mountains down to the valleys and out to the sea [מלבי״ם, רד״ק, שטיינזלץ]. The water recedes at a perfectly even and steady pace, much like a person walking with unvarying strides [קונטרס חיבה יתירה]. This slow, natural withdrawal stands in sharp contrast to the rapid and violent eruption of water that began the flood [קאסוטו].
As the waters decline, the reduction is not merely a lowering of the surface level but a complete disappearance from certain areas, leaving places like mountain peaks entirely dry [שד״ל]. Another perspective suggests that the waters decreased on their own, almost as if the water was naturally absorbing itself [אור החיים].
A discussion arises regarding exactly when this decline begins, centered around a period of one hundred and fifty days. The primary approach among commentators is that the waters remained at their peak and only started to recede after these one hundred and fifty days concluded, a date calculated as the first of the month of Sivan [רש״י, מזרחי, גור אריה, שפתי חכמים, הדר זקנים, דעת זקנים]. Some add that this precise rate of descent was not a natural process but a miraculous occurrence directed by God [העמק דבר]. In contrast, other commentators argue that water remaining entirely stagnant for months after the rain stopped would defy the laws of nature. They maintain that the waters began to lessen the very moment the rain ended. However, it took one hundred and fifty days for the decrease to reach a significant enough stage for the ark to finally rest on the mountains [שד״ל, ביאור יש״ר, הטור הארוך].
This timeline is also explained through Noah's personal experience. As long as the ark was floating, Noah could not notice the dropping water levels, since he was surrounded entirely by endless water and sky. The decrease only became apparent and felt by him after one hundred and fifty days, when the ark finally struck solid ground [מלבי״ם, קאסוטו, בכור שור]. Regarding how the exact count of days was known, the accepted understanding is that these precise dates and calculations were either communicated to Noah through a prophecy from God, or they were revealed much later to Moses when he wrote the Torah. Noah himself simply knew to wait and check the condition of the earth after he felt the ark come to a complete stop [אבן עזרא, קרני אור, יהל אור].