SOIL SURVEY

-161-Table 65 . Guides for assessing soil susceptibility to water erosion .The evaluations of soil susceptibility to water erosion are based on the assumption that natural geologic water erosion is expected and accepted . Thus, th epredicted erosion potential applies only when man's activities (including fires) cause a loss of vegetation on the soil surface which leads to accelerated erosio nof that soil surface . It is also assumed that most of the activities which occur within the Park will not cause a disturbonce which is deep enough to penetrat ebelow the sedum . Accordingly, the predicted susceptibility to erosion applies only to the surface 25 to 50 centimeters of the soil .The susceptibility to water erosion of the mapping units (Table 67) was evaluated by means of the graph which Is presented in Figure 107 .The soil ratings, K, (Table 65a were estimated by the method of Wischmeier, Johnson and Cross 0971), (Figure 107) and the slope angle ratings were adopted i npart ham Rutter(1968) . The divisions on the graph are arbitrary, and are based on observations and published data .Field observations made during the soil survey indicate that in Yoho there are two main exceptions to the foregoing procedure for estimating susceptibility to wate rerosion . The soil erodibility factor, K (Figure 107) is a poor estimate of erodibility in lime rich horizons (Ck, Cca) or dense till materials . These materials ar egenerally found below the solo. and were not considered in the susceptibility to erosion ratings . Where the erosion potential of the parent materials or lime cemente dtills is of interest, it con be evaluated by Rutter's (1968) method, using soils information which is presented in this report .The second exception occurs when soils contain appreciable quantities of coarse fragment s ) (greater than 2 mm) . Coarse fragments aren't evaluated by the soi lerodibility factor, but the problem is partially evaluated by the dashed lines in Figure 108 .Table 65a . Soil credibility ratings (K values) determined usin gFigure 107 .Mop Unit K Value % Coarse Fragment s 2PROCEDURE : Wbr,aeee+ob 10 .10.3 .0 mml, , 400,Mll uerce. b nn pbuNn i n JOpueMe l;SP, uM S%,OM }.Bt,u k+ue ], pnmeWeuv!SOb un . •O.JI .Figure 107 . Soil-erodibility nomograph (token Pram Wischmeier, Johnson, and Cross 1971) .Soils with greater than 20% coarsefragments (C .F . = 2 mm to 25 cm )are less susceptible to erosion an dthe band between the dashed linesIndicates moderate erosion risk i nsuch cases .K values are poor estimates ofcredibility in Ck or Cca horizonsespecially in tills which are alsodense, and estimates made fro mthis figure exclude lime-cemente dhorizons .Figure 108 . Erosion hazard of soils .70SK-352K•2000%Lo werosionrisk,hsodera t \hNihs! nos rOnrisk` \HigherasionriskLow Moderate 30Y. High80%IA01 (EF) (GPOSLOPE ANGLE RATIN GBC 1 0 .50 < 2 0BC 2 0 .60 < 20BC 3 0 .60 < 20BG2 0 .25 >2 0BG 3 0 .50 > 20BG4 0 .45 >20B G 5 0 .55 < 20CM1 0 .30 20DS 1 0 .45 < 20FL 1 0 .35 > 20H01 0 .60 < 20HR 1 0 .20 < 20HR 2 0 .45 < 20K11 0 .60 < 20K12 0 .60 < 20M01 0 .35 > 20NI1 0 .40 < 20OD 1 0 .30 < 20OG 1 0 .30 < 20OG 2 0 .35 < 20OH 1 0 .25 > 20011 0 .40 < 20OL 2 0 .45 < 20001 0 .30 < 20002 0 .30 < 20OP 1 0 .20 >20OT 1 0 .35 < 20SI1 0 .40 >20SI2 0 .35 20WR2 0 .40 >20WR3 0 .55