Preliminary Report
Sedimentation Survey of Galena River Valley, Wisconsin - Illinois
Stafford C. Happ
Sedimentation Division
Soil Conservation Service
Washington, D.C.
14 May 1940
SCS Region 5
8 - Upper Mississippi
Middle Mississippi Watershed
A sedimentation survey of the Galena River Valley, and tributaries, was initiated in June 1939, under an informal cooperative arrangement between the Sedimentation Division, Soil Conservation Service, and the Wisconsin State Experiment Station. Mr. Clifford Adams, a graduate student at the University of Iowa, was employed for the month of June by the Soil Conservation Service to make such a survey, with the understanding that he would complete the survey and make a laboratory study of certain selected sediment samples by methods of mechanical and mineralogical analysis, at his own expense as a thesis study under the direction of Prof. A. C. Trowbridge.
Necessary instrumental surveys, to provide data needed for interpretation of the data gathered by Mr. Adams, are being carried out by the Sedimentation Division, which also supervised and inspected the field work. Not all the filed data has yet been collected, and hence a full report on the survey cannot yet be made, but preliminary data have been compiled in order to make this information available to the U. S. Engineer Office, Rock Island, Illinois, at the request of the District Engineer, for use in connection with a flood-control survey currently under way by that office.
The field procedure was to make test borings, with Iwan-type soil augers, at varying intervals along range lines crossing the flood plain approximately at right angles to the long axis of the valley. At each boring hole, the depth of sediment overlying the soil horizon, which was at the surface prior to settlement of the region and consequent accelerated sedimentation due to soil erosion and washing of mining debris into the valleys, was measured and recorded. The width of the modern sediment deposits, and distance between borings, was measured by chaining. Ground-surface profiles were run along the range lines, and tied to standard elevation datum, and areas of sedimentation, of stream-bank erosion, and of obvious land damage by infertile overwash were mapped on aerial photographs. Range lines have been established at approximate intervals of one mile throughout the length of the main Galena River Valley, and three ranges each have been established in four main tributaries. Additional test borings are still to be made on some of the ranges, and additional ranges are also to be established on some tributaries.
The cross-sectional area of modern flood plain sediment at each range has been determined by planimeter measurement on the plotted cross-sections. The areas of sedimentation were obtained by planimeter from aerial photographs on which the margins were mapped in the field. From these data sediment volumes were computed by the mean end-area formula,
V = (A/3) * { [(E1 + W1)/(E2 + W2)] + [E1/W1] + [E2/W2] }
There is computed to be 9,387 acre-feet of modern sediment covering 4,389 acres on the flood-plains of the main Galena River and four major tributaries, East Fork, Hughletts Branch, Coon Branch, and Shullsburg Branch. With allowance for the deposits in other tributaries, the total sediment accumulation in the entire drainage basin, during the 120 years or so since white settlement, is estimated to be about 15,230 acre-feet, covering about 8,139 acres. The present and immediate future rate of sedimentation is probably higher than the average for the entire 120-year period, but data are not yet adequate to provide more accurate evaluation of present rates.
Where alluvial fans have been recognized in the course of the survey, these have been mapped and estimates of the thickness have been made from auger borings and field exposures. As these are not adequately represented in the volumes computed from valley cross-sections, they have been compiled and are listed separately in the accompanying tables. Silt fans are computed to cover about 63 acres, with a volume of about 126 acre-feet, and gravel fans cover about 23 acres; with about 46 acre-feet. The gravel fans are derived chiefly from mine waste. These data are only for alluvial fans in the main valley and in four principal tributaries covered by the survey.
The greatest average thickness of sediment, 5.25 feet, was found at range 33, located about 400 feet south of the Illinois Central Railroad bridge at the southern margin of the city of Galena. The average thickness is nearly 4 feet from here to Galena Junction, where the Galena River enters the Mississippi flood plain, and about 4 feet from Galena up to the mouth of the East Fork (Range 29). Above the mouth of the East Fork the average thickness is nearly 2 feet to range 21 near the Illinois-Wisconsin State line, and from that point to the head of the flood plain, about four miles east of Platteville, it is about 1-1/2 feet.
In the city of Galena, between ranges 32 and 33, most test borings failed to locate the old flood-plain soil horizon, and hence the depths of modern deposits could not be measured reliably. This condition is believed to be due to extensive channel changes and artificial modifications incident to former navigation improvements. Successful borings were made at either margin of the urban area, however, and these show deposits averaging about 5 feet deep and with a cross-sectional area of about 2,900 square feet. This deposit has reduced by a corresponding amount the cross-sectional area available for discharge of overbank floodwaters, and probably has raised flood stages for certain discharge volumes some 2 to 3 feet. The exact extent of this effect on flood heights varies of course, for different stages, and will generally be less for higher stages because of the wider valley section available at the higher stages.
Except at Galena, where flood damage is due to overflow of the improved urban properties, it does not appear that increased flood heights due to flood-plain sedimentation have been an important factor in flood damage. This is partly because there are very few low terraces in the Galena Valley, which would be affected by rising flood heights and resulting more frequent overflows.
Data are being gathered for evaluation of stream-bank erosion as a source of sediment and a cause of land damage, but observed field conditions suggest, in comparison with conditions in other Wisconsin drainage areas where surveys have been made, that the total volume of sediment production by stream-bank erosion has not been more than 10 percent of the volume of modern sediment accumulation.
Laboratory studies have been undertaken by Mr. Clifford Adams, under supervision of Professor A. C. Trowbridge at the State University of Iowa, in an effort to evaluate the contribution to the the valley deposits from mining wastes. The total volume of sediment in the valleys, however, does not appear to be relatively greater than in other Wisconsin valleys where there has been no contribution from mining waste.
No effort has yet been made to gather data on the volume of sediment carried out of the Galena drainage basin as suspended load. In other areas where conditions are generally similar, available data indicate that less than half the sediment reaching the valley has been carried into the Mississippi River or Mississippi flood plain.