When did Yitzchak re-dig the wells that the Pelishtim had stopped up before expelling him from Gerar?
Ramban (to 26:17): These are different wells, located in a town called 'Nachal Gerar,' where Yitzchak was now staying, not in Gerar; or in a section of the River of Gerar, far away from Gerar itself. This was not under the direct jurisdiction of the King of Pelishtim, but under that of another king. 3 After Avraham died, the residents stopped up the wells that he had dug there - but not out of hatred. 4
Refer to 26:15:2:1. Gur Aryeh - If they sealed the wells only now, after Yitzchak left, how could Yitzchak name them after the Pelishtim had sealed them off?!
The Ramban queries this however, inasmuch as the Pelishtim would hardly have granted him permission to do so. Refer to 26:18:1.1.
Malbim: Avraham dug them on his return to Be'er Sheva from Eretz Pelishtim. He delayed in Nachal Gerar, like shepherds do.
Why did Yitzchak call the wells by the same names as his father had?
Rashbam: So that nobody would be able to claim that the wells belonged to them. Ha'amek Davar - this is because no one contested them in the days of Avraham.
Malbim: All actions of the Avos and their ascent from level to level, they made markers of them below in the lower world. When they brought down a spiritual influence from the river that exudes from Eden, they dug wells and called them names that reflect exalted matters in the upper worlds. Pelishtim stopped them up, for they thought that they draw from their Sar above. Yitzchak re-dug them, and drew water of Chesed in the merit of Avraham, and returned the conduits of Berachah to their place.
Because he followed in the footsteps of his illustrious father and therefore merited Divine protection on his merit, too (though not on his own merit). 1 (EC)
Why did the Pelishtim stop up the wells only after Avraham's death?
Ramban (to 26:17): It was due to his honor, lest he return.
Oznayim la'Torah: This reflects upon the deep respect that they had for Avraham Avinu - that, as long as he was alive, they would not do so, even though he had long before moved to Chevron!
QUESTIONS ON RASHI
Rashi writes: "'He went back and re-excavated them." Why did Yitzchak do this, if the wells had been sealed to thwart invasions by enemies (Rashi to 26:16)?
Gur Aryeh #1: At the time, there had been a war. Now the war had ended, so Yitzchak re-opened the wells.
Gur Aryeh #2: Yitzchak felt that their justification was merely an excuse, and their real reason for sealing the wells was jealousy. Now that Yitzchak had left, there would be no more jealousy.
Gur Aryeh #3: The wells were in Yitzchak's current location, the valley of Gerar. As long as Yitzchak was there, there was no concern that enemy bands would use them.