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Sheet and Gully Erosion in the Missouri Valley LoessialRegion
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STREAMS that drain the loessial
lands bordering the lower MissouriRiver have always carried sizeablequantities of sediment (1) ". Thesesediments are largely derived by sheeterosion from upland areas and bycyclic erosion activity in gullies anddrainageways. This erosion is bothnatural and manmade.
The interaction of natural erosive
forces with those imposed by man isnot well understood. Daniels and Jor-dan (2) postulate that the streamtrenching and gullying which beganabout 1880 in Harrison County, Iowa,for example, could have been relatedto natural cyclic factors. There is littledoubt, however, that cultivation hasincreased runoff and erosion rates andprolonged this gullying cycle, becausestream trenching has continued to thepresent.
The purpose of this paper is to as-say the erosion problem in the MissouriValley Loessial Region by quantifying,first, the erosion in those upland fieldsand gullies where much sediment orig-inates and, second, the sediment trans-ported to downstream locations (sedi-ment yield). Data from past and pres-ent studies are then utilized to explainsome variations in erosion rates andsediment yields, in terms of some af-fecting variables.
SHEET EROSION AND SEDIMENT YIELD
Sheet-rill erosion rates (hereaftertermed "sheet erosion" or "soil loss")have been measured for a variety ofcropping and management conditionsat experiment stations in northwesternMissouri and western Iowa. Many ofthese .small-plot erosion measurementshave been incorporated into empiricalexpressions (3, 4) that relate soil lossto characteristics of rainfall, soil, to-pography, land use, and land manage-ment. These existing formulas are thebasis for much land-resource planning.
Paper No. 67-703 presented at the WinterMeeting of the American Society of AgriculturalEngineers at Detroit, Mich., December 1967, ona program arranged by the Soil and Water Di-vision.
The authors-R. F. PlEST and R. G. SPOMER-are hydraulic engineer, USDA, ARS, SWC,Columbia, Mo., and agricultural engineer, USDA,ARS. SWC, Council Hluffs, Iowa.
"Numbers in parentheses refer to the ap-pended references.
Authors Note: This paper is a contributionfrom the North Central Watershed Research Cen-ter, Corn Belt Branch, SWCRD, ARS, USDA,in cooperation with the Iowa and Missouri Agri-cultural Experiment Stations.
R. F. Piest and R. G. SpomerMEMBER ASAE Assoc. MEMBER ASAE
TABLE 1. SEDIMENT YIELDS FROM STREAMFLOW SAMPLING
Station Drainage,acres
Recordlength,years
Sedimentyield,
tons per acre per year
Mule Creek near Malvern, IowaTarkio River at Blanchard, IowaWest Tarkio Creek near Westboro, MissouriThompson Creek near Woodbine, IowaSteer Creek near Magnolia, Iowa
6,780128,000
67,2004,2805.950
13%6644
3.75.24.84.23.5
The records from these field-plot stud-ies, furthermore, show negligible lossesfor many good-management plots andimmense sheet erosion rates for storms
occurring on vulnerable soils at criticalcrop stages. For example, single-stormerosion rates at Bethany, Missouri (5)and Castana, Iowa (6) have been inexcess of 20 tons per acre, and annuallosses frem sheet erosion have exceeded
60 tons per acre on row crops.Sediment yields at downstream loca-
tions are related -to the quantities ofsediment produced and the efficiencyof transport of sediment derived up-stream. The ratio of sediment yield tototal material eroded on the contribut-ing watershed is termed the sedimentdelivery ratio. Sediment delivery isdten expressed as a fraction or per-centage of total upstream ero:1ed soil.Although the onsite erosion rates frcmfield-size watersheds with long, steepslopes can far exceed those measureden small experimental plC!ts (7), manvfactors operate to reduce the sedimentyield, per unit drainage area, at down-stream points.
In the Missouri Valley loessial area,most of the soil particles are in the silt-clay size range and can be moved witha minimum velocity of the entrainingrunoff. Therefore, the most significanttransport losses occur during the sheet-erosion process, when soil is moved insheets and micro-channels, and largelocal changes in flow gradient are pos-sible. Deposition is usually most preva-lent at the base of steep field slopes.Once in a graded channel or gully,there is little opportunity for these finesediments to deposit.
Sediment yields for some watershedsin the deep loess region have been de-termined from reservoir surveys andfrom streamflow sampling programs.Summaries from about 50 small reser-
voir watersheds in the loess region (8,9, 10) show only a few sediment yieldsthat average more than 20 tons peracre per year. This approximates theupper limit for land continually row
cropped without conservation treat-ment.
Streamflow sampling programs, in-tended to show sediment dicharge ra~esand sediment yields, are usually of shortduration but encompass a wide rangeof watershed sizes. Typical sedimentyields from small loessial watersheds inthe area - those with the longest gag-ingilnd sampling experience-arelisted in Table 1.
These sediment quantities seem ac-ceptable at first glance; they are belowthe 5 to 6-tons-per-acre annual maxi-mum that conservationists allow in ero-sion-control design for this area. Fur-ther study, however, reveals that manyparts of these watersheds are noncon-tributing because dams and terraceseffectively control sediment movement;the principal contributory areas arecultivated upland fields, with land slopesto 20 percent, which are presently usedbeyond their capabilities. Much ofthe soil eroded from these fields is notflushed through the drainage system.
A better focus on sheet erosionsources in the Missouri Valley loess canbe obtained by considering the resultsfrom intensive studies on ARS water-sheds near Treynor and Macedo~ia,Iowa. These watersheds, located anddescribed in Fig. 1, have been gagedsince 1964. This period is too short forconclusive findings, but some progresshas been made in evaluating land-useand trealment effects as well as shestand gully erosion phenomena.
Annual sediment yields for ARS wa-tersheds, from both sheet and gullysources, are shown in Fig. 2, along withconcurrent sediment yields measuredat Mule Creek near Malvern, Iowa( 10.6 sq miles), Steer Creek near Mag-nolia, Iowa (9.3 sq miles), and Thomp-son Creek near Woodbine, Iowa (6.7sq miles). The paired contour-corn wa-tersheds 1 and 2 experienced muchgreater soil loss (averaging nearly 30tans per acre per year) than eitherthe grass (W -3) or the level-terraced(W-4) watershed. These differences
This article is reprinted from the TRANSACTIONSof the ASAE (Vol. 11, No.6, pp. 850, 851, 852 and 853, 1968)Published by the American Society of Agricultural Engineers, St. Joseph, Michigan
" -----------------------.\ POTTAWATTAMIECOUNTY,IOWA !
I II I
( ?J.JJJ I"-, MILES I\ I
... T~nor I~C;I Bluffs I(W-4 W-3 W-~ I--, W-2-/W-I ./ J~---~--------------
WATERSHED
4
FIG. 1 Location and description of research watersheds.
in sediment yield were especially sig-nificant in 1964 and 1965, when annualrainfall averaged 6 and 16 in., respec-tively, above the 29-in. norm. Sedimentyield from both sheet and gully sourceswas 1 ton per acre or less on grasseJwatershed 3 and level-terraced cornwatershed 4. The sheet erosion rates
between terraces were significantlyhigher, but nearly all the eroded soilwas retained in the level-terrace chan-
nels. Confirming the erosion controleffected by terraces, the measured sedi-ment yielJ from the 339-acre, level-terraced, mixed-crop watershed nearMacedonia, Iowa, was 1 ton per acreor less.
These sediment yields from single-crop or single-treatment watersheds arealso compared (Fig. 2) with those fromthree nearby watersheds having variedcropping and treatment. As would beexpected, the sediment yields for Mule,Steer, and Thompson Creek watershedsare intermediate in magnitude.
The variation in sediment yield fromactive sheet erosion sources is perhaps
40
lruJ20
wa: 0()<t
1964
~~,.,
~ 00 ~U'
1965
rJ) 60z0f- 40
rJ)
8 20
--' nil,.,--' 0arJ)
g 4O
Loj:§ 20<t
0,W-I W-2 W-3 W-4 W-5 MULESTEER THOMPSON
, -,- ,--TREYNOR MACEOONIA CREEKS
1966
FIG. 2 Variation in sediment yields forwatersheds with diiIerent land use-:rea.-ment histories.
exemplified by the erosion history ofwatersheds 1 and 2, which are field-contoured and ~ in continuous corn,with an average land-surface slope of8 to 10 percent. It was possible toseparate sediment yield according. tosource - sheet-rill erosion or gullyerosion - by sampling streamflowabove and below the raw gully head-cut at the outlet of each watershed.
Fig. 3 summarizes the effect of therainfall variable, 1964-66, when annualprecipitation fqr each of the 3 years wasabnormal.
A 20-ton per acre per year sedimentyield from sheet erosion appears to benear the long-term normal for mis-managed cropland in this soil area, butthe within-year and year-to-year rain-fall variation" is so large that a "normal"erosion year is unlikely.
Other characteristics of sheet-erosion
phenomena are evident from a study ofTable 2, which includes the storm sedi-ment yields from sheet erosion sourcesfor 1965 and 1966. More data are
needed before precise relationships aredrawn, but the 2-year experience showsthat, although sediment yield fromsheet erosion sources is runoff-entrainedand is correlated with runoff amount
and intensity, it is still highly variable.The overall runoff-sediment relation
(not shown) indicates that, on theaverage, each inch of runoff might beexpected to move about 3 tons of soilfrom each acre of cornfield and throughthe drainage system. Note, however,that the 1.9-in. total rainfall of May22, 1965, produced less than 1 inchof runoff but more than 8 tons per acreof sediment from the drainage systemsof watersheds 1 and 2.
Findings from Table 2 also revealthat soil loss equations, which are usu-ally reliable predictors of long-termsoil losses from field areas, do not pre-dict individual-storm soil losses to the
rsame accuracy. Comparisons betweencomputed storm soil loss and measuredsediment yields from sheet-rill sourcesshow excessive sediment delivery, es-pecially during the early crop-stage pe-riods. Sediment delivery at watersheds1 and 2 during May and June 1965,for example, was quite high. The meas-ured sediment yields sometimes ex-ceeded the computed storm soil losses.By contrast, the sediment delivery aver-aged about 2{) percent for the Septem-ber 1965 storms.
These excessive sediment yields andhigh-delivery ratios during the earlycrop-stage periods must be the resultof conditions and processes that arenot encountered in the usual applica-tion of the soil-loss equation to com-pute long-term yields. Two causes are:
1 Antecedent moisture conditions,which vary from storm to storm withinseason. The soil-loss equation assumesaverage antecedent moisture from allstorms.
2 The development of large, com-plex rill systems, especially in tilled,unsodded drainageways.
GULLY EROSION
Gullying is prevalent throughout mostof the United States, and Bennett (11)estimated that 200 million active gullieswere in existence in 1939. Gullies voidfarmland, dissect fields, impede efficienttillage operations, create safety hazards,and contribute to downstream sediment
problems. They are particularly trou-blesome in the deep loessial soils bor-dering the entrenched Missouri Riverand its major tributaries.
Brice ( 12) described the essentialfeatures of a gully to be its size (mustbe larger than a rill), recency of ex-tension in length, steepnessof head andsides, incision into unconsolidated ma-terials, and ephemeral transmission of
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40
OJ351;'"a:"'~30'"~
~2~"g'"~20
ffi...OJ
~ 15'"0ff:c 10oJ>'...ffi .'"~
".. 0
(--) NOR"ACmcu>""'o". 's.. '"c,,,'--
0
015 40 4020 25 30 35
ANNUAL PRECIPITATION. INCHES
FIG. 3 Average relationship between pre-cipitation and sediment yield, field-con-toured com watersheds 1 and 2, 1964-66.
WATERSHED DESCRIPTION
SIZE CROPPING LAND TREATMENT
ACRES
74.5 Con';nu,,"s Corn F;eld-Contoured
82.8 Cont;nuous Corn Field- Contoured
107 Gro.. None
I O Continuo". Corn Le.el- Te"oced
389 M;,.d Le.el-Te"oced
TABLE 2. HYDROLOGIC AND SEDIMENT INFORMATION FOR ALL MAJOR STORMS ON WATERSHEDS I AND 2, 1965-1966
flow. Gully cutting can begin at anylocation in a drainageway where gradi-ents or local disturbances permit con-centration of runoff. Gully growth isthen continued by upstream movementcf the overfall, channel degradation,and lateral enlargement of the channel.
The quantities of sediment movedfrom gully systems in loessial regionsare highly variable. Brice ( 12) , instudies of gully erosion in the deeploess of western Nebraska, found frommeasurements on aerial photos that the216 active upland gullies in the 20-square-mile Dry Creek Watershed con.tributed 66 acre-ft of sediment in the15-year period, 1937 to 1952. This is6 percent of the estimated average an-nual sediment yield (13). In addition,erosion of the main channels by en-largement and headcutting accountedfor a large amount of sediment.
Dvorak and Heinemann (14) found,from channel surveys and a stream-flow sampling program, that the IIIacre-ft of sediment eroded from the
main valley-bottom gullies of Dry Creekconstituted 68 percent of the measuredsediment load passing the streamflow-sampling station during the above-normal rainfall period, May 1951 toMay 1952. For the dry years, 1952 to1958, surveys showed that only 14.6acre-ft was eroded from valley-bottomgullies, and this constituted a muchsmaller portion of the total sedimentload. On the average, however, gullyerosion accounts for 10 to 30 percentof the total sediment yield from thosesmall watersheds where active gullyingis not inhibited by remedial measures.
In western Iowa, Beer ( 15) esti-mated the 1942 to 1962 gully erosionin the 9-sq-mile Steer Creek watershedto be 10.5 tons per acre annually. This
rate dropped to 2 tons per acre peryear in 1962-1963 and averaged lessthan 2 tons per acre per year, 1964through 1967. Gottschalk and Brune( 8) used reservoir sedimentation sur-veys and aerial photo measurements asa basis for their conclusion that gulliescan be the major sediment contributoron some watersheds.
Many factors relating to gully forma-tion and growth have been identifiedbut need further quantification. In theloessial region under consideration,Thompson (16) found that the squareroot of watershed size was best corre-lated with gully growth. Beer andJohnson (17) included an index of sur-face runoff and the distance from gullyhead to watershed divide in an equa-tion to estimate areal changes in agully with time; these are also corre-lated with watershed size. Leopold etal (18) cite the moisture in the sub-
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\<.'_-: \. I ,
I I ;I ;, I ;
-<::::-.=--~ /: /
(:.~.=~==..~.:.~.~~~.~.../"; 1 "."::,,.. "
FIG. 4 Runoff and accompanying gullygrowth during 1965, watershed 1 nearTreynor, Iowa.
stratum and several other factors, butconclude that "surprisingly little isknown, at least quantitatively, aboutthe mechanics of the process."
Erosion rates from raw gullies lo-cated at the outlets of watersheds 1 and2 are listed in Table 2', columns 5 and11, for most storms in 1965 and 1966.In 1965 alone, 1150 tons was erodedfrom the vicinity of the gully head ofwatershed 1, whereas only 230 tonswas removed from the gully head andchannel immediately below the head-cut of watershed 2. The data for wa-tershed 1 are of special interest be-cause the extremely large quantities ofmaterial removed during the periodmust be near the upper limit of gullyerosion that is possible for the givenenergy inputs. For this reason, theserecords probably offer the best oppor-tunity for identifying the unattenuatedbasic interactions responsible for gullyerosion; once identified, complementarymodel experiments can be designedthat will further isolate and evaluatethese erosive forces.
Fig. 4 locates the moving gully headof watershed 1 at different times dur-ing the high runoff year of 1965 andshows the runoff quantities associatedwith this erosion. Although the exactrole of runoff in gullying is not known,the storm runoff component is the prin-cipal mover of gully debris. It is re-sponsible for bed and bank scour byvirtue of the tractive forces exerted; itcauses localized scour by plunge-poolaction at the headcut; it is responsiblefor some bank saturation and sloughing-activities. Other contributory forces re-sult from wetting of gully banks byrainfall, seepage from ground water,subsequent wet-dry, freeze-thaw cycles,and the effect of gravity.
Watershed 1 Watershed 2
Sediment yield Sediment Sediment yield SedimentComputed d Ii Computed d]"
Date Rainfall, Runoff, From heet From f(Ully total sheet b:sed Rainfall, Runoff, From sheet From gully total sheet b e :;deryinches inches erOSIOn erOSIOn erosion, sheet inches inches erosion erosion erosion as Ont:r s:e, tons per erosion, source source, tons pe; sh,ettons per tons erOSIon,acre acre percent acre acre percent
1965April 4, 5 1.13 0.32 1.63 40 0.66 247 1.13 0.42 1.80 40 0.47 383April 8 1.01 0:10 0.21 32 0.82 26 1.09 0.06 0.11 17 0.88 12May 17, 18 1.12 0.20 3.80 30 5.07 75 0.88 0.12 2.15 8 2.39 90May 21 0.87 0.25 2.93 77 2.68 109 0.89 0.23 2.49 112 2.32 107May 21, 22 1.92 0.95 9.64 223 3.08 313 1.88 0.96 8.01 116 3.07 261May 24 0.77 0.28 3.01 32 2.37 127 0.65 0.25 2.14 15 1.20 178May 25, 26 0.47 O.ll 1.21 15 0.57 213 0.41 O.ll 1.09 5 0.98 IIIJune 2 0.48 0.03 0.12 10 0.63 19 0.34 0.03 0.08 1 0.24 33June 6 0.64 0.37 2.39 115 1.70 140 0.78 0.36 2.05 75 1.44 142June 28, 29 4.24 1.59 14.00 97 25.30 55 4.02 1.61 11.30 52 17.40 65July 1 0.93 0.18 0.97 37 0.96 101 0.82 0.18 0.74 14 0.72 103July 30 0.76 0.03 0.09 3 0.80 1<1 0.81 0.05 0.10 3 0.87 12Aug. 29, 30 1.89 0.23 0.32 43 4.55 ' 7 1.74 0.23 0.51 21 3.19 16Sept. 7 4.28 1.15 1.07 134 9.80 II 3.97 1.03 0.94 94 7.78 12Sept. 8 1.17 0.23 0.25 40 0.66 38 0.99 0.15 0:13 2 0.36 36Sept. 17, 18 0.55 0.06 0.04 8 0.45 9 0.51 0.04 0.03 1 0.30 10Sept. 18 0.52 0.06 0.07 12 0.18 39 0.91 0.12 0.17 6 0.56 30Sept. 20 1.82 0.33 O.ll 68 1.06 10 1.92 0.28 0.07 10 1.12 6Sept. 27 0.86 0.07 0.04 17 0.46 9 0.95 0.06 0.04 3 0.43 9Sept. 29, 30 0.96 0.09 0.09 25 1.82 5 0.90 0.08 0.07 5 0.4'2 17
1966May 15 0.53 0.03 0.42 5 1.37 31 0.65 0.10 1.81 22 1.07 169May 23 0.50 0.01 0.03 1 0.75 4 0.48 0.01 0.03 1 0.32 9June 5 0.88 0.08 1.01 19 3.60 28 0.88 0.12 2.18 0 2.53 86June 25, 26 2.44 0.46 5.19 67 8.71 60 2.76 0.50 4.51 161 8.25 55July 14, 15 1.62 0.01 <.01 1 4.20 0 1.91 <.01 <.01 0 3.28 0
DATES RUNOFF GULLYEROSION
A",-Ft. To"--- Jo.,-Ap,,4 29 130---- ""'15-J"e9 17 510.---------- J"eIO-Ao,.13 32 160- Ao,14-Dee 8 25 350
Totol 103 1150
The process of gully cleanout duringthe course of the storm of May 25,1965, and the erosive effect of the run-off are shown in Fig. 5. The continu-ous concentration curve of gully ero-sion was obtained by gaging the runoffand by collecting 33 streamflow sam-ples above and below the gully headof watershed 1. In this way, it was pos-sible to separate the sediment concen-tration (and discharge) into sheet ero-sion and gully erosion components.
At the outset of the storm, the sedi-ment concentrations were near maxi-mum, but rapidly decreased before therunoff rate reached a maximum. Soonthereafter, from 2116 to 2121 hr, thesupply of soil debris in the channel wasexhausted. The later increase in sedi-ment concentration is attributed to mas-sive slumping of the gully bank at theheadcut.
SUMMARY
The limits of upland erosion rates inthe Missouri Valley Loessial Region-from sheet and gully sources have beengenerally defined for several types ofland use on the basis of measurements
from experiment station plots, reser-voirs, and streams. For the highlyerodible loessial soils bordering thelower Missouri Valley, small-plot~oiLlosses from poorly managed 'row cropsIlaVe ex~eeqed 60 tons per acre perye~r and 20 tons per acre p~.r ~JOrm.
Long-term sediment yields from...field;size:lI§"~.sshould generally ,Eot.§,.x;;.2eed ~ ,tol}sB..e.r ac.!.e annually Jor field-contoured row crops; this estimate isbased on interpretation of reservoirsedimentation surveys and measure-ments at ARS watersheds near Trey-
~"o,000
g
~ :::::>-~ ,,~oo~~00 ",000
~0
"'° 'TIME
FIG. 5 The process of gully cleanout andthe erosive effect of runoff, storm of May25, 1965, watershed 1.
nor, Iowa. By comparison, losses fromlevel-terraced cornfields or from mead-
ows 'were~s}~a~~ ttuiii IJoii .p~racre annually'. S~aiment yields from~ro§ion ~!Iq gW,bLP9$10
