Genesis 26:17

And Izhak went thence, and sojourned in the vale of Gerar, and dwelt there.

IN THE VALLEY OF GERAR, far away from the city. And [Isaac] digged again the wells of water, which they had digged in the days of Abraham his father, and which the Philistines had stopped up before Isaac left Gerar, he once again dug. Thus the words of Rashi.
It would appear from Rashi’s words, then, that the wells mentioned here [in Verse 18] are the same as those mentioned [above in Verse 15]: For all the wells which his father’s servants had digged in the days of Abraham his father the Philistines had stopped them, and filled them with earth. But this is not so, for how would the Philistines give him permission when they had said, “These wells are a source of danger to us because of invaders.” Rather, the wells [mentioned in Verse 18] are different ones in another location since the valley of Gerar is the name of a place, or perhaps the valley extended from Gerar to another land, and when the Philistines, namely, the lords of Gerar — the principality of the king — became jealous of Isaac, they stopped the wells which were his by inheritance from his father in the boundary of the city of Gerar. The king then sent him away from his throne-city, and he went to another city. Possibly, this new place was not part of his kingdom even though it was in the land of the Philistines. There were other wells there which Abraham had dug when he lived there for many days but which the Philistines stopped upon his death since Isaac did not dwell there. They did not do this out of hatred. But in the days of Abraham, [even after he had moved from there], they did not want to stop them out of respect to him, saying, “Perhaps he will return to dwell there.” This was why Isaac returned and dug them in the valley. But the herdsmen of the valley quarrelled with him saying, The water is ours, meaning, “The well is in the valley, and the waters ooze forth from that valley while those in the valley are diminished. Hence they are ours.” It is for this reason that Scripture mentions, And they found there a well of living water, stating that it was a source gushing forth living water which did not come from the valley as those who quarrelled with Isaac had previously claimed.

וילך..בנחל גרר. The word נחל describes a valley even though there was no river in that valley. We have another example of this in Deuteronomy 2,36 העיר אשר בנחל, “the city which was located in the valley,” or Samuel II 24,5 ימין העיר אשר בנחל, “to the right of the city in the valley.” The most potent proof for our statement is Genesis 26,19 ויחפרו עבדי יצחק בנחל, “the servants of Yitzchok dug in the valley.” If there had been water, a rivulet, or something in the valley, what did they have to dig for? Gerar was the name of the town in that valley, the place where Yitzchok settled. He dug many wells in that region as he owned much livestock and had to provide water for his beasts.