Physiographic History and the Soils, Entrenched Stream Systems, and Gullies, Harrison County, Iowa

Abstract

Cyclic erosion and sedimenta.tionhave helped determine the present landscape in Thompson Creek and ).[agnolia watersheds in Harrison County, Iowa.The major configuration of the landscape such as posi tion of valleys and drainage divides probably was determined in pre Wiscon"ill time.In the 15,000 years since deposition of the Tazewell loes;; mantle, there have been at least five periods of alluviation on the valley floor.The fir;;t two require(; about 13,000 years for completion.Eac-h of the three pedods completed in the last 2,000 years was preceded by erosion of the older "fill in some part of the drainage system.Some erosion wa" ('hannel treneliing.Some valley-;;lope erosion was independent of Rtream erosion.The resulting geomorphic: surfaces of different ages form a complex pattern on the valley slopeR and the valley floor.The ;;oilf> tlis('ussed in this bulletin formed in Tazewell loess but are of dilTerent ages because of nllley-siope erosion.The soil properties vary with agf' and slopt'o Past and prC'sC'nL stream tren!'lling in these watersheds indicates that, if the s(reamf> are not ("OIl trolled, the modern trem'hing eyele will end in the same fashion as the eyt'ie that preeeded it.Filling followed cutting in tlU' earlier <"yc'ies.O[H'e tIl(' s\n'ams are at grade, alluviation again seems likely.Eventually, the streams will flo'\" on a nanow flood plain between banks sC'pantting the httest fill from the older fills.Erosion is not likc,ly lu destroy the alluvial (ill.\YatC'lsheds that have a complex ('tit-and-fill history apparently ha\'e a high potential for stream trenching and tlwir use should bC' planned ll('('ordingly.Cully formation probably is part of the normal cycle of landscape evolution in the thick friable loess of west em Iowa.The end result of gullying in this aren is nn extension of the drainage system.Growth of individual val10y-slope gullies cnn bC' stopped by directing water elsewhere by praeli('es now common in the area.They should present no serious prohl01ll to ('ontinued agric'ultural usc.iii ACKNOWL.EDGMENTSThis bulletin reports a 1957-1960 study of landscapes, soils, entrenched streams, and gullies in parts of Harrison County, Iowa, supported by Soil Survey Investigations, Soil Conservation Service.The major ob jectives w/}re to determine the Cll.uses and effects of stream entrenchment and gully formation in the thick-loess area of western Iowa and to evaluate the influence of geomorphic proccsses and physiographic history on the genesis, geography, and classification of soils.The senior author acknowledges the constructive criticisms of R. V. Ruhe, research geologiflt, and Guy D. Smith, director, Soil Survey In vestigations, Soil Conservation Service, during