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1707 Figure 10 near here. Median seasonal dissolved and total-recoverable iron concentrations in the npper Arkansas River, April 1990-March 1993 Figure 11 near here. Median percentage of dissolved iron in the upper Arkansas River, April 1990-March 1993 Table 10 near here. Average percentage of seasonal instantaneous streamtlow and instantaneous total-recoverable iron load contributed' to the downstream main-stem site in a reach by the upstream main-stem site and the intervening tributaries in the same reach, April 1990-March 1993 Dissolved and [oral-recoverable iron concentrations decreased significantly at the LMDT during the post- treatment period, but concentrations at California Gulch exhibited no significant change (table 11). Nonpoint-source loading of iron to California Gulch from sources other than the Yak Tunnel probably contributed a substantial amount of iron and were unaffected by the treatment plant. Iron concentrations at most main-stem Arkansas River sites were not statistically different during the pre-treatment and post-treatment periods (table II). The results of the etatistica! tests of pre-treatment and post-treatment concentrations were not unexpected owing to the wide distribu- etion of iron-enriched mine tailings and the ubiquitous nature of iron throughout the basin. Table 11 near here. Results of Wilcoxon rank-sum test (I-tailed) ofpre-treatrnent (April 1990-March 1992) and post-treatment (April 1992-March 1993) instantaneous streamtlow and iron concentrations in the Arkansas River . '~!:'::~:'!":c::';!'~B.;",~.. [1""'" .... . --,""0 x, u:" .;J,1r~ l!;~;;. '~.-:." .."',." ..':' -. --... -.. 'WJ~..dfg.:a. b~';":.;if-. j Subject to Ravision . ;':''-0 NOT QUOTE OR RE1~,~~:c r::;r;;:!ing Aoorovol by DI'r-e":.~::'. U S ." .. . ...~l.,j.I, ., . G~o:or:col Sur..€'y , I TT