Why does the CBP border zone nearly touch Reno, Nevada?

Question

The CBP is authorized to do citizen checks within 100 miles of the US border. The ACLU shows this on a map, here.

Now, Lake Michigan counts as a national border, probably because of the Boundary Waters Treaty of 1909, which allows Britain (Canada) unlimited navigation of all five Great Lakes.

But look at the mid-west coast. What makes the 100-mile limit lurch inland, nearly touching Reno, Nevada? Is there a national border that runs up the Sacramento River delta, perhaps to Sacramento or Stockton?

And if Sacramento, why doesn't it run up the Mississippi River?

Answer 1

The immediate explanation is the substantial eastward jog created by Suisun and Grizzly Bay. This would follow from 33 CFR 2.20, defining the territorial sea baseline, following the Convention on the Territorial Sea and the Contiguous Zone. The zone would fall just short of Lake Tahoe. It's based on tidal waters, see 33 CFR 329.12 for similar definition of the US baseline in terms of waterbodies subject to tidal action.

Answer 2

My (uneducated) guess would be that whoever made this graphic simply took the "outline" of the United States on this map, and colored red anything that was within 100 miles of this "outline". This outline happens to include some bays on the coasts, like the San Francisco Ban or the Chesapeake Bay, as indentations, so the red area extends further inland in those areas. The Mississippi River and the Sacramento River delta happen to not be depicted as indentations on this map, so the algorithm did not draw the red area based on those. I am not sure that the red area on this map is a legally accurate depiction of the 100-mile border zone.