by E. Arthur Bettis III
A gully is a relatively deep, vertical-walled channel, recently formed within a valley where no well-defined channel previously existed. Western Iowa gullies range from five to over 80 feet in depth and from three to 100 feet in width. Some gullies are several miles long while others are as short as 100 feet. All have nearly vertical walls and contain streams which have extreme variations in discharge throughout the year. Gullies in large valleys such as Keg Creek and Silver Creek contain streams which usually flow year round, but streams in most gullies are dry during portions of the year.
Gullies develop because of a decrease in the erosional resistance of the land surface or an increase in the erosional forces acting on the land surface. What causes gullies to form, when and where they do is poorly understood. Field and laboratory studies indicate that certain reaches of a valley are more prone to gully development than others. However the timing of the initial downcutting and which of the "most probable" reaches develops into a gully cannot be predicted with certainty.
Once a gully has formed, the processes whereby it lengthens and widens are much better understood. The upper end of a gully is marked by a headwall, a vertical scarp, separating the ungullied portion of the valley floor from the gully below. Water flows over the headwall during runoff and falls into a plunge pool at the base of the headwall. The water then erodes the base and sides of the pool, undercutting the headwall. When undercutting reaches an advanced stage the headwall fails and topples into the gully, thereby lengthening the trench. This process is repeated many times as a gully advances up the drainageway.
When first formed, most gullies are quite narrow and have vertical sidewalls. Increased pore pressure from groundwater moving toward the gully, coupled with some undercutting of the sidewalls causes deep rotational slumps along the sidewalls. If enough water is flowing through the gully to carry away the slumped material, additional slumping can occur. This causes the gully to widen. Widening also occurs when upper portions of gully walls separate and topple into the gully. This phenomena is most common following heavy spring rains and during freeze-thaw cycles in the late winter and early spring. If water intermittently flowing through the gully continues to clean out debris derived from the headwall and sidewalls, the gully continues to grow. When more debris accumulates than is transported away, the gully stabilizes and begins to fill.
Numerous researchers have pointed the finger at agriculture as the cause of western Iowa's gully problems. Specifically, they cite the increases in runoff that result from land clearing, overgrazing, cultivation, and stream channelization. Numerous federal, state, and county agencies spend millions of dollars annually to control existing gullies and promote land management practices which reduce runoff in an attempt to alleviate the gully problem.
Many of today's gullies are cut into alluvium, the sediment transported and deposited by flowing water in streams. In most of western Iowa, the source of the alluvium is the silty loess found on valley slopes. Vertical gully walls, such as those shown in the accompanying photograph, often expose several distinct layers of alluvium. Layers of similar sediments can be traced within a single valley and also can be recognized from one valley to the next, a process called correlation. Six distinct layers, or alluvial fills, can be recognized in small valleys throughout western Iowa. Extensive core drilling in these valleys and interpretation of exposures formed along gully walls prove that these alluvial fills accumulated in old gullies.
Occasionally, buried tree stumps, logs, or charcoal are found enclosed in these old alluvial fills where they are exposed along modern gully walls. These organic remains have been radiocarbon-dated and a chronology of gully cutting and filling constructed. More than 100 such radiocarbon dates indicate that the six major alluvial fills recognized in western Iowa valleys represent regionally synchronous episodes of gully cutting and filling during the last 12,000 years. Four of these episodes occurred during the last 4,000 years, and the deposits associated with them are rather well preserved and understood.
About 3,500 to 4,000 years ago, deep gullies much larger than today's dominated the landscape in small western Iowa valleys. In many cases these gullies occupied the entire valley floor. Beginning shortly after 3,500 and continuing until about 2,000 years ago, gully growth stopped and alluvium accumulated in the gullies. By 2,000 years ago the gullied areas were completely filled with silty sediment washed from the adjacent valley slopes, and marshy areas occupied the central portion of the former gullied areas.
Sometime during the 200-year period between 2,000 and 1,800 years ago another gully cycle began. Gullies extended up all moderate-sized valleys and some of their lateral tributaries. Gullying did not extend into small drainages at the upper end of the drainage network as it had during the previous cycle. In extent, depth, and width of gullying, this cycle is analogous to modern gullying in the area. Shortly after 1,800 years ago alluvium again began to accumulate in the gullies, eventually filling them by about 1,000 years ago.
The third gullying cycle began about 800 years ago. In this cycle, gullying was restricted to moderate-sized and larger valleys and did not extend as far up valleys or into smaller valleys as it had during either of the previous episodes. These new gullies were restricted to central portions of the area gullied during the previous cycle. Further, these gullies were not as deep or as wide as earlier gullies had been. Shortly after the gullies developed they began to fill with alluvium. Sediment accumulated until the gullies were completely filled and portions of the surfaces bordering the gullies were buried a few feet. Counts of growth rings in trees growing on alluvium filling these gullies indicate that sedimentation may have continued until about 100 years ago.
The most recent western Iowa gully cycle began around 100 years ago. Numerous accounts in local histories, original land surveys and early reports of the Iowa Geological Survey indicate that until about 1860 gullies were not widespread in the area. By 1900 reports of problems arising from gully growth, such as the need for bridges at crossings, became common and indicated that the historic period of gully growth was in full swing. In some valleys, gullies have formed and been filled several times during this historic cycle, a process which also occurred during the prehistoric episodes but is too obscure to be interpreted from the geologic alluvial-fill record.
The geologic record contained in western Iowa valleys shows that major gullying is not new to the area. Several episodes, some more widespread than that which affects the area today, occurred prior to Euroamerican settlement and the spread of modern agriculture. Gullying is part of the natural process of landscape evolution in western Iowa. The modern gullying which causes so much concern is also part of this natural process. No doubt, landuse changes accompanying the spread of agriculture and urbanization have aggravated and possibly accelerated the growth and extension of gullies in western Iowa. However, the geologic record suggests that the area was "due" for an episode of gullying prior to the 1850s. Gullies grew and filled several times in the past when humans were not significantly influencing runoff or vegetation patterns. This indicates that human activity affects gullies in this area but does not cause them.
Recognition of the fact that gullies are "native" to western Iowa is important because it indicates that gullies are not a unique phenomena resulting entirely from human modification of the landscape. Through recognition of gully-prone valley sections and the promotion of landuse aimed at preventing or lessening the factors causing gullies in those areas, we can avoid gully growth or lessen its impacts. During the last 12,000 years, gullies and the erosion resulting from their growth have molded the western Iowa landscape into that which we see today. This process is active and will continue to be so far into the future. Currently our knowledge of the factors contributing to gully initiation is very incomplete. Somewhat better understood are the processes and factors involved in gully growth and degradation. These are areas of urgent research needs. Through a better understanding of the processes affecting gully growth and filling, we can lessen the impact our activities have in promoting the gully problem and plan around those portions of the gully network which are too costly or not likely to be controlled.
Author's Note: Dean Thompson, Natural Resources Conservation Service, is acknowledged for his considerable contributions to our understanding of Iowa gullies. Thompson and Bettis have collaborated extensively, collecting data used to interpret the history presented in this article.
Adapted from Iowa Geology 1983, No. 8 Iowa Department of Natural Resources