Question: What river was featured in a film serial starring Helen Gibson, and was the site of the Battle of Crow Agency in 1887?

Context paragraphs:
Little Bighorn River: The Little Bighorn River is a 138 mi tributary of the Bighorn River in the United States in the states of Montana and Wyoming.  The Battle of the Little Bighorn also known as the Battle of the Greasy Grass was fought on its banks in 1876, as well as the Battle of Crow Agency in 1887.

Crow War: The Crow War, also known as the Crow Rebellion, or the Crow Uprising, was the only armed conflict between the United States and the Crow tribe of Montana, and the last Indian War fought in the state.  In September 1887 the young medicine man Wraps-Up-His-Tail, or Sword Bearer, led a small group of warriors in a raid against a group of Blackfoot which had captured horses from the Crow reservation.  Following the raid, Sword Bearer led his group back to the Crow Agency to inform the Indian agent of his victory, but, an incident arose which ended with the young leader taking his followers into the mountains.  In response, the United States Army launched a successful campaign to bring the Crow back to the reservation.

Zorro's Black Whip: Zorro's Black Whip is a 1944 Republic Pictures film serial starring Linda Stirling.  The film was made after the 1940 20th Century-Fox remake of "The Mark of Zorro" and Republic wasn't able to use the character himself, but still wanted to capitalize on it.  However, and despite the title, Zorro does not feature in this serial.  The hero(ine) is actually called The Black Whip throughout.

The Tiger's Trail: The Tiger's Trail is a 1919 American adventure film serial starring Ruth Roland, directed by Robert Ellis, Louis J. Gasnier and Paul Hurst.  A "fragmentary print" from the serial survives.

Custer's Last Stand (serial): Custer's Last Stand is a 1936 American film serial based on the historical Custer's Last Stand at the Little Bighorn River.  It was produced by the Poverty Row studio Stage & Screen Productions, which went bust shortly afterwards as a victim of the Great Depression.  This serial stars many famous and popular B-Western actors as well as silent serial star Helen Gibson.

Haunted Valley: Haunted Valley is a fifteen episode American adventure film serial starring Ruth Roland, in which Ruth Ranger, the president of an engineering firm engaged in a construction project at the Lost River Dam, takes out a three-month loan million dollar loan from supposed friend Harry Mallinson with the "Haunted Valley" as collateral.

Sea Raiders: Sea Raiders is a 1941 Universal film serial starring the Dead End Kids and Little Tough Guys.  This was the teen stars' second of three serials, between "Junior G-Men" (1940) and "Junior G-Men of the Air" (1942).  "Sea Raiders" was the 52nd serial to be released by Universal (or the 120th if silent serials are counted).  The plot concerns the heroes foiling Nazi attacks on American shipping.

The Tiger Woman (1944 film): The Tiger Woman (1944) is a 12-chapter Republic film serial starring Allan Lane and Linda Stirling (her serial debut).  The serial was re-released in 1951 under the title Perils of the Darkest Jungle and, in 1966, it was edited into the 100-minute Century-66 film Jungle Gold.

Gloria's Romance: Gloria's Romance was a 1916 silent film serial starring Billie Burke.  Serial films, also called chapter plays, were shorter films that were typically run before the main feature film, each of which was part of a longer story, and ended in a cliffhanger, thus encouraging the audience to return every week.

Tarzan the Fearless: Tarzan the Fearless (1933) is a 12 chapter American Pre-Code film serial starring Buster Crabbe in his only appearance as Tarzan.  It was also released as a 71-minute feature film which consisted of the first four chapters of the serial version.  Actress Jacqueline Wells co-starred; she later changed her name to Julie Bishop.  The serial was produced by Sol Lesser, written by Basil Dickey, George Plympton and Walter Anthony (based on the character created by Edgar Rice Burroughs), and directed by Robert F. Hill.  The film was released in both formats on August 11, 1933.

Supporting paragraphs: Custer's Last Stand (serial) Little Bighorn River
Answer: Little Bighorn River