12
Opinion of the Court
ory that these will deplete the tributaries' contributions to the mainstem, and hence upset "the equitable balance of the North Platte River established in the Decree," id., at D-5, D-10. Wyoming's second exception takes issue with the Master's stated intention to consider a broad array of downstream interests in passing on Nebraska's claims, and to hear evidence of injury not only to downstream irrigators, but also to wildlife and wildlife habitat. Third Interim Report 14, 17, 19, 26.
Consideration of this evidence, Wyoming argues, would run counter to our denial of two earlier motions to amend filed by Nebraska: its 1988 motion, 485 U. S. 931, by which it expressly sought modification of the decree to make Wyoming and Colorado share the burden of providing instream flows necessary to preserve critical wildlife habitat, and its 1991 motion, see 507 U. S. 1049 (1993), in which it sought an apportionment of nonirrigation-season flows. Wyoming also suggests that allegations of injury to wildlife are as yet purely speculative and would be best left to other forums.
Wyoming's arguments are not persuasive. To assign an affirmative obligation to protect wildlife is one thing; to consider all downstream effects of upstream development when assessing threats to equitable apportionment is quite another. As we have discussed above, Nebraska II makes it clear that modification of the decree (as by enjoining developments on tributaries) will follow only upon a "balancing of equities," 507 U. S., at 592, and that Nebraska will have to make a showing of "substantial injury" before we will grant it such relief, id., at 593. There is no warrant for placing entire categories of evidence beyond Nebraska's reach when it attempts to satisfy this burden, which is far from insignificant.
Nor does our resistance to Nebraska's efforts to bring about broad new apportionments (as of nonirrigation-season flows) alter this conclusion. Here, Nebraska seeks only to have us enjoin discrete Wyoming developments. If Ne-
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