The Arkansas River Project

The Arkansas River flows for almost exactly 400 miles through Kansas, from the Colorado line at Coolidge to the Oklahoma line south of Ark City. Over this course the river drops significantly in altitude as it flows towards to gulf, dropping off its cargo of alluvially transported rocks and gravel on its way across the plains. It is this payload that we as rockhounds are interested in, of course, since it contains the best of the gems, agates, and fossils that we try to find on the muddy gravel bars across the state. This project was an attempt to collect at least a minimum of data regarding the river and its current alluvial transporting capabilities, as well as simply learning more about it through a survey of sites across the state.

Zooming out a bit from our focus of Kansas, it is helpful to consider the entire course of the upper Arkansas River as it rushes down from the Rockies towards the High Plains. The graph below shows this descent by averaging the rate of the elevation loss for each short segment of river examined (25-40 miles each).

This data is of course a bit imprecise, having been collected manually from Google Earth data, but is representative of the riverbed to within a few meters. As can be seen, the descent is quite steep as the river exits the mountains, dropping nearly 600 meters in the first 75 miles. Following this rapid descent the river then slows to a more leisurely pace of around 2 meters per mile for the next few hundred miles, a remarkably straight course for such a long river.

We can visualize this same data in a more intuitive fashion by plotting the actual elevation of the river in a line chart as seen below, and gain a better sense for the slope of the Great Plains.

If we zoom in a bit on this graph and examine the portion of the river bounded by Kansas, we can see a bit more detail.

As we can see, the river drops nearly 750 meters in its course through Kansas, but follows a remarkably consistent rate of elevation loss, only flattening slightly near the southern reaches of the state.

It is probably not possible to draw significant hypothesis about the deposition of alluvial gravels from this data alone, particularly since conditions and flow rates may have been far different when most of our gravel was deposited (regardless of the assumed age of the sediments, no further Rocky Mountain alluvium has reached us in the 80 years since the construction of two reservoirs and respective dams in Colorado). It is tempting to look for a sudden drop or plateau that would indicate the rapid deposition of quantities of river load, but no such large-scale drops exist.

It was assumed from this data that a likely similar linear decrease in alluvia mass as the river progresses across the plains, and so a largely unscientific experiment was proposed to test this hypothesis. While it is impossible to accurately assess the largest stone size at any one point within a river basin, it is safe to assume that if the largest few stones at any given point on the river are averaged and then plotted vs. distance a similarly defined curve should appear. For our methodology we chose to collect only felsic rocks (those consisting primarily of Orthoclase Feldspar), since these were of assured igneous origin, and thence must originate from the Rocky Mountains.

My team adopted this method, and points were chosen at roughly 30 mile intervals from the western edge of the state down to the Oklahoma border. We started the experiment in June of 2021 in far western KS in the town of Syracuse (we attempted to collect samples closer to the state line, but high water kept us from finding samples). We were able to collect samples of the largest stones at each site all the way back to central Kansas in Edwards county on this trip, and have plotted the results we gained below.

The data is clearly incomplete (and really almost anecdotal from a strictly scientific perspective), but it is certainly interesting. This is as far as the project has brought us so far, since I am away at school most of the year and can't continue gathering data, but we look forward to gathering new insights about the deposition of the transported stones we seek.

We filmed a significant portion of our first survey, and have posted the vlog of the survey on youtube at the link below. Watching it should give you a better sense of our methodology and motivation behind trying to see what's out there and catalogue it to the best of our abilities.