[1][5] The park is roughly 17 miles (27 km) long, following the course of the Genesee River as it flows north through a deep gorge and over several large waterfalls.

In 1859, Buffalo industrialist William Pryor Letchworth (1823–1910) began purchasing land near the Middle Falls, and started construction of his Glen Iris Estate.

Activities within the park include hiking, biking, fishing, whitewater rafting and kayaking, geocaching, and hunting (wild turkey and deer when in season).

[8] The three major waterfalls — called the Upper, Middle, and Lower Falls — are located in Portage Canyon, the southern section of the park.

The historic, restored Glen Iris Inn, William Pryor Letchworth's former residence adapted for use as a hotel, is located on the top of a cliff overlooking Middle Falls and offers in-season meals and overnight accommodations.

Her remains were exhumed from the Buffalo Creek Reservation and reinterred on the grounds of a Seneca Council House, relocated to the site by Letchworth and rededicated in 1872.

[12] Operated year-round by New York State, the 5,000-square-foot (460 m2) sustainable building features classrooms and meeting rooms, a research lab, a butterfly garden, and connections to various trails.

[13] Construction of the Mount Morris Dam, at the north end of the park, was begun in 1948 by the United States Army Corps of Engineers under the Flood Control Act of 1944, and completed in 1954.

[14] The Genesee River became wider and deeper immediately upstream as a result, but areas downstream were spared yearly flooding which destroyed valuable farmland.

The Mount Morris Dam is the largest flood control device of its kind (concrete gravity) east of the Mississippi River.

[17] On November 29, 2011, Norfolk Southern Railway announced plans to demolish the Portage Viaduct and build a new bridge approximately 75 feet (23 m) to the south of the 1875 structure.

The territory of the park was long part of the homeland of the Seneca people, who were largely forced out after the American Revolutionary War, as they had been allies of the defeated British.

[6] Having first viewed the gorge that was to become the park from a train on the nearby railroad trestle in 1858,[6] William Pryor Letchworth purchased an initial 190 acres (0.77 km2) of land near the Portage Falls in 1859[2] and subsequently began work on his Glen Iris Estate.

[6] He enlisted the services of the famous landscape artist William Webster to design winding paths and roadways, rustic bridges, glistening "lakes" and a sparkling fountain.

[6] George F. Kunz, president of the American Scenic and Historic Preservation Society of New York, was an enthusiastic supporter of the park.

The hurricane then went out to sea via North Carolina, recharged its energy and hit the Southern Tier of New York State on June 22, 1972.

Video footage of the devastation can be viewed upon request at the William Pryor Letchworth Museum, located near the Glen Iris Inn.