Since its inception in 1890, the U.S. Board on Geographic Names has discouraged the use of the possessive form — the genitive apostrophe and the "s". The possessive form using an "s" is allowed, but the apostrophe is almost always removed. The Board's archives contain no indication of the reason for this policy.
However, there are many names in the GNIS database that do carry the genitive apostrophe, because the Board chooses not to apply its policies to some types of features. Although the legal authority of the Board includes all named entities except Federal Buildings, certain categories — broadly determined to be "administrative" — are best left to the organization that administers them. Examples include schools, churches, cemeteries, hospitals, airports, shopping centers, etc. The Board promulgates the names, but leaves issues such as the use of the genitive or possessive apostrophe to the data owners.
Myths attempting to explain the policy include the idea that the apostrophe looks too much like a rock in water when printed on a map, and is therefore a hazard, or that in the days of "stick-up type" for maps, the apostrophe would become lost and create confusion. The probable explanation is that the Board does not want to show possession for natural features because, "ownership of a feature is not in and of itself a reason to name a feature or change its name."
Since 1890, only five Board decisions have allowed the genitive apostrophe for natural features. These are: Martha's Vineyard (1933) after an extensive local campaign; Ike's Point in New Jersey (1944) because "it would be unrecognizable otherwise"; John E's Pond in Rhode Island (1963) because otherwise it would be confused as John S Pond (note the lack of the use of a period, which is also discouraged); and Carlos Elmer's Joshua View (1995 at the specific request of the Arizona State Board on Geographic and Historic Names because, "otherwise three apparently given names in succession would dilute the meaning," that is, Joshua refers to a stand of trees. Clark's Mountain in Oregon (2002) was approved at the request of the Oregon Board to correspond with the personal references of Lewis and Clark.
I've always wondered about the aversion to apostrophes. My parents recently lived in a town called Scottsbluff, which is named after a nearby bluff named after a man named Scott.
Not only is the apostrophe missing, but so is the space, which makes me want to say it with primary stress on the first syllable and no stress on the second, when in fact it's pronounced with secondary stress on the first and primary stress on the second.