even the slopes of the wadis
The cited lines refer to the slopes of the wadis reaching toward Ar and leaning on the border of Moab.
This verse fills out the geographic memory attached to the border.
10Then the Israelites set out and camped at Oboth. 11They journeyed from Oboth and camped at Iye-abarim in the wilderness opposite Moab to the east. 12From there they set out and camped in the Valley of Zered. 13From there they moved on and camped on the other side of the Arnon, in the wilderness that extends into the Amorite territory. 14Now the Arnon is the border between the Moabites and the Amorites. Therefore it is stated in the Book of the Wars of the LORD: 15even the slopes of the wadis 16From there they went on to Beer, the well where the LORD said to Moses, “Gather the people so that I may give them water.” 17Then Israel sang this song: 18The princes dug the well; 19From the wilderness the Israelites went on to Mattanah, and from Mattanah to Nahaliel, and from Nahaliel to Bamoth, 20and from Bamoth to the valley in Moab where the top of Pisgah overlooks the wasteland.