Hydrologic investigations of prairie potholes in North Dakota, 1959-68

Abstract

A prairie pothole is a depression in the prau1e, capable of storing water, that is the result of glacial processes. Years ago, there were many hundreds of thousands of prairie potholes in the North-Central United States, but large numbers of them have been drained for agricultural use. This report is limited to studies of prairie potholes in the eastern part of the glaciated northern Great Plains region in North Dakota-a rolling upland area covered with glacial drift, called the Coteau du Missouri. Potholes are wetlands that are the primary breeding area of migratory waterfowl in the United States. If production of waterfowl is to continue, suitable wetlands must be maintained, and even new wetlands created to offset those destroyed for agricultural use. The initial stage of the Garrison Diversion Unit calls for a normal annual diversion from Garrison Reservoir of 60,000 acre-feet of water for this purpose. Many prairie potholes contain large amounts of emergent aquatic vegetatjon known as hydrophytes. Determining the loss of water by transpiration from emergent hydrophytes was one of the major objectives of the present study of the hydrology of prairie potholes. Other hydrologic factors were studied later, but the first part of the study was devoted almost exclusively to the determination of evaporation and transpiration losses at groups of potholes in Ward, Stutsman, and Dickey Counties. The mass-transfer method was used, and by determining the variation in the mass-transfer coefficient throughout a season, the losses by evaporation and transpiration were determined separately. Separate determinations were accomplished by relating the emergent height and the moisture content of the hydrophytes to the rate of transpiration, as determined by the mass-transfer coefficient. Seasonal evaporation from the study potholes clear of vegetation was found to very nearly equal the generalized evaporation values published by the U.S. Weather Bureau. The effect of hydrophytes in potholes was twofold: their presence reduced evaporation from the water surfaces; and, at the height of the growing season, their transpiration rate, added to the reduced evaporation rate, frequently was greater than the evaporation rate from potholes clear of vegetation. Net seepage outflow from potholes was generally very small-less than 0.01 foot per day per unit of water surface. This rate of seepage was not insignificant, however, because it often amounted to more than one-fourth of the total seasonal .loss of water from a pothole. The source of water supplying the evapotranspiration losses was primarily precipitation on the water surface of a pothole pond. Augmenting this supply was basin inflowoverland flow, flow in channels, and seepage inflow. Of these, overland flow was estimated to have been the largest by far; direct observations were not possible. Basin inflow was very erratic; it depended on combinations of events, such as antecedent soil moisture and rainfall intensity, or depth of snow at time of melting and concurrent rainfall. The occurrences of these combinations were such that, for a given season (October-March or April-September), the total basin inflow generally showed little relation to total precipitation. The greatest inflows were associated with late snowmelt flowing over frozen ground. Following the evapotranspiration study, the effects of ground-water movement were investigated. All the study potholes were located in areas of glacial till in order to reduce the effect of seepage in the mass-transfer computations. Accordingly, ground-water movement was not a major factor in the water budget of the study potholes; however, it could be, in potholes located in areas of outwash sands and gravels, and in potholes in glacial till with only temporary ponds. Also, the direction of ground-water movement has a controlling effect on the water quality of a pothole pond. Where there is no seepage outflow or overflow, there is no mechanism for the removal of dissolved solids brought to the pond by basin inflow. Such potholes are saline-some even more so than sea water. Conversely, potholes that receive no seepage inflow generally contain fresh water and are usually not permanent. All conditions between these two extremes were found. The permanence of water in a pothole (the extent to which the water body is permanent) and its quality were found to have a direct and significant relation to the species of vegetation that grows under those conditions. In fact, the species of vegetation are excellent indicators of water quality and permanence, and the report contains a table listing the common species used as indicators and the conditions that they indicate. Many other facets of the hydrology of prairie potholes were investigated to ensure that no major factor was ignored, and the investigation results are described briefly.