A Very Considerable River

Clark's Map of the Marias River

Detail based on Clark-Maximilian Sheet 25
Maximilian-Bodmer Collection, Joslyn Art Museum, Omaha, Nebraska

On Sunday, June 2, 1805, the Corps of Discovery camped on the bank of the Missouri opposite the mouth of "a very considerable river." It was too late in the evening for them to study the terrain in detail, so they put that off until morning. The next day, Clark calculated the widths of the two rivers.

First he measured a distance between two objects or stakes on the near shore; the distance had to be long enough to make a "strong" angle to the opposite shore (generally between 30 and 60 degrees). Clark then took a bearing between the two objects on the near shore and a bearing to a convenient object (tree, rock, etc.) on the far shore. He then went to the opposite end of the baseline and shot a bearing from there to the object on the far shore. That done, he made a scale drawing and determined the width of the river by measuring the scaled plot at right angles to the near-shore baseline, with allowances for mud flats, islands, or sand bars. He calculated that the left fork was 362 yards wide. The right fork, which Lewis was soon to name Maria's River, was 200 yards wide, or 55.2% of the Missouri's span.1

The year 1805 was near the end of the Little Ice Age, a period of cooler, wetter weather that affected the Northwest beginning in the 1400s, and came to a close around 1850. The two rivers probably were near flood stage following a winter of comparatively deep snow.

Hydrogeologist Robert Bergantino, of the Montana Bureau of Mines and Geology, has scaled the widths of the two rivers by a means that Lewis would have envied. Using the most recent United States Geological Survey map, made from aerial photos taken in the late summer of 1952, Professor Bergantino plotted a series of five different measurements on each river beginning about 1/4 mile upstream from the junction. The resultant average for the Marias was 95, which was 54.2% of the average for the Missouri, 175 yards.

Although water levels on the Missouri River have been controlled for many years to prevent major flooding, and despite the impacts that irrigation and other human interference have imposed on the two rivers, their proportionate widths are almost exactly the same today as they were in 1805.