Rivers & Floodplains
Blog Entry: December 21, 2010
Much of the geology of Whitman Mission is inextricably tied to the rivers and creeks that flow through and around it. With the exception of the memorial shaft hill (which we'll talk about next week), the park is strikingly flat. This is because we are located near the confluence of two of the area's most important water bodies: the Walla Walla River and Mill Creek. More specifically, Whitman Mission is located at the point where the floodplains of these two streams coalesce. If you were to dig a few feet down into the ground at just about any point in the park, you would find sand and gravel deposited by these two streams during major floods over the past few thousand years.
One especially interesting and historically significant geological feature at the park is a dry "oxbow" of the Walla Walla River. "Oxbow" is the term for a U-shaped curve in a stream that gets cut off from the main channel. In wetter climates, oxbows usually fill with water and are called "oxbow lakes", but the one here at Whitman Mission is mostly dry. When the Whitmans first arrived here in 1836, the Walla Walla River passed directly through what is now Whitman Mission National Historic Site.
Rivers such as the Walla Walla, which originates in a nearby mountain range and can carry lots of sediment during periods of flood, can change their course surprisingly quickly. It can take as little as one major flood to fill the river channel with sediment and force the river to overflow its banks and carve a new path across the landscape. Over the past 150 years, a combination of natural changes such as this and changes brought about by agricultural practices in the area have forced the Walla Walla River into a new channel several hundred yards to the south of Whitman Mission, leaving the old, dry oxbow behind.
When the Whitmans arrived in 1836, Dr. Whitman built their first house immediately adjacent to the banks of the Walla Walla River. While the location of the house was convenient for washing and gathering water, had Dr. Whitman been a geologist, he most certainly would have selected a better site for his house. Located less than 10 yards from the bank of the river, the First House was extremely prone to flooding. In 1838, the Whitmans began construction on a new house farther from the banks of the Walla Walla on ever so slightly higher ground. It was also here along the river that the Whitmans' two-year old daughter, Alice Clarissa, drowned in 1839.