For the second time in three years, we confront a dispute between Florida and Georgia over the proper apportionment of interstate waters.
Florida, the downstream State, brought this original action against Georgia, claiming that Georgia consumes more than its fair share of water from an interstate network of rivers. Florida says that Georgia's overconsumption harms its economic and ecological interests, and it seeks a decree requiring Georgia to reduce its consumption.
When the case was last before the Court, we resolved it narrowly and remanded to a Special Master with instructions to make fndings and recommendations on additional issues. Florida v. Georgia, 585 U. S. ––– (2018). On remand, the Special Master recommended that we deny Florida relief for several independent reasons, including that Florida proved no serious injury caused by Georgia's alleged overconsumption.
Based on our independent review of the record, we agree with the Special Master's recommendation. We therefore overrule Florida's exceptions to the Special Master's Report and dismiss the case.
I
This case concerns the Apalachicola-Chattahoochee-Flint River Basin, an area spanning more than 20,000 square miles in Georgia, Florida, and Alabama. The Basin contains three *Briefs of amici curiae were fled for the United States by Acting Solic itor General Wall, Deputy Solicitor General Kneedler, Deputy Assistant Attorney General Grant, Frederick Liu, and Michael T. Gray; for the Atlanta Regional Commission et al. by Lewis B. Jones; and for Franklin County, Florida Seafood Workers by Lisa S. Blatt.
Page Proof Pending Publication rivers. The Chattahoochee River and the Flint River start in Georgia and empty into Lake Seminole, which straddles the Georgia-Florida border. Both rivers are critical sources of water for Georgia. The Chattahoochee is the primary water supply for the Atlanta metropolitan area, while the Flint supplies irrigation to southwestern Georgia's agricultural industry.
The third river in the Basin is the Apalachicola River. It starts from the southern end of Lake Seminole and fows south through the Florida Panhandle, emptying into the Apalachicola Bay (Bay), near the Gulf of Mexico. The Apalachicola River supports a wide range of river wildlife and plant life in the Florida Panhandle, and its steady supply of fresh water makes the Bay a suitable habitat for oysters. For many years, Florida's oyster fsheries were a cornerstone of the regional economy.
Many factors infuence Apalachicola River fows, including precipitation, air temperature, and Georgia's upstream consumption of Basin waters. The U. S. Army Corps of Engineers also plays an important role. The Corps regulates Apalachicola fows by storing water in, and releasing water from, its network of reservoirs in the Basin. In recent years, low fows in the Apalachicola River have become increasingly common during the dry summer and fall months, particularly during droughts.
In 2013, on the heels of the third regional drought in just over a decade, Florida brought this original action against Georgia, seeking an equitable apportionment of the Basin waters. See 28 U. S. C. § 1251(a); U. S. Const., Art. III, § 2. Florida asserts that Georgia's overconsumption of Basin waters causes sustained low fows in the Apalachicola River, which in turn harm its oyster fsheries and river ecosystem. As a remedy, Florida seeks an order requiring Georgia to reduce its consumption of Basin waters. Florida does not seek relief against the Corps.
Page Proof Pending Publication We granted Florida leave to fle its complaint and referred the case to Special Master Ralph Lancaster, Jr. After 18 months of extensive discovery and a 5-week trial, the Special Master issued a report recommending that Florida be denied relief. Although the Special Master assumed for the sake of his analysis that Florida had suffered serious injuries due to Georgia's upstream water use, he determined that it was unnecessary to make defnitive fndings on those issues because Florida failed to prove by clear and convincing evidence that any remedy would redress its asserted injuries. That was so because a remedial decree would not bind the Corps, which could operate its reservoirs to offset any added streamfow produced by the decree.
On review of Florida's exceptions to the Special Master's Report, we remanded for further proceedings. Florida v. Georgia, 585 U. S. –––. We concluded that the Special Master's clear and convincing evidence standard for the “ `threshold' ” question of redressability was “too strict,” at least absent further fndings. Id., at ––– – –––. We then directed the Special Master to make defnitive fndings and recommendations on several additional issues, including: whether Florida had proved any serious injury caused by Georgia; the extent to which reducing Georgia's water consumption would increase Apalachicola River fows; and the extent to which any increased Apalachicola fows would redress Florida's injuries. Id., at ––– – –––.
Soon after our decision in Florida, Special Master Lancaster retired, and we appointed Judge Paul Kelly as the Special Master. Following supplemental briefng and oral argument, Special Master Kelly issued an 81-page report recommending, for several independent reasons, that this Court deny Florida relief. Relevant here, the Special Master concluded that Florida failed to prove by clear and convincing evidence that Georgia's alleged overconsumption caused serious harm to Florida's oyster fsheries or its river Page Proof Pending Publication wildlife and plant life. Second Report of Special Master 8–25.
Florida again fled exceptions to the Special Master's Report. We must “conduct an independent review of the record, and assume the ultimate responsibility for deciding all matters.” Kansas v. Nebraska, 574 U. S. 445, 453 (2015) (internal quotation marks omitted). Having done so, we overrule Florida's exceptions and adopt the Special Master's recommendation.
II
“This Court has recognized for more than a century its inherent authority, as part of the Constitution's grant of original jurisdiction, to equitably apportion interstate streams between States.” Id., at 454. Given the weighty and competing sovereign interests at issue in these cases, “a complaining State must bear a burden that is `much greater' than the burden ordinarily shouldered by a private party seeking an injunction.” Florida, 585 U. S., at –––.
Here, Florida must make two showings to obtain an equitable apportionment. First, Florida must prove a threatened or actual injury “of serious magnitude” caused by Georgia's upstream water consumption. See id., at –––, ––– (internal quotation marks omitted); Colorado v. New Mexico, 459 U. S. 176, 187, n. 13 (1982) (Colorado I). Second, Florida must show that “the benefts of the [apportionment] substantially outweigh the harm that might result.” Id., at 187. Because Florida and Georgia are both riparian States, the “guiding principle” of this analysis is that both States have “an equal right to make a reasonable use” of the Basin waters. Florida, 585 U. S., at ––– (emphasis deleted; internal quotation marks omitted).
To resolve this case, we need address only injury and causation. Florida asserts that Georgia's overconsumption of Basin waters caused it two distinct injuries: the collapse of its oyster fsheries and harm to its river ecosystem. Florida does not dispute that it must prove injury and causation by Page Proof Pending Publication clear and convincing evidence. See Colorado I, 459 U. S., at 187, n. 13. To do so, Florida must “place in the ultimate factfnder an abiding conviction that the truth of its factual contentions are `highly probable.' ” Colorado v. New Mex ico, 467 U. S. 310, 316 (1984) (Colorado II).
With Florida's heavy burden in mind, we address its asserted injuries in turn.
A
In 2012, in the midst of a severe drought, the oyster population in the Apalachicola Bay collapsed, causing commercial oyster sales to plummet. By the time of trial, the Bay's fsheries had yet to recover. All agree that this is an injury “of serious magnitude” under our equitable-apportionment precedents. See New York v. New Jersey, 256 U. S. 296, 309 (1921).
The parties, however, offer competing explanations for the cause of the collapse. Florida pins the collapse on Georgia through a multistep causal chain. It argues that Georgia's unreasonable agricultural water consumption caused sustained low fows in the Apalachicola River; that these low fows increased the Bay's salinity; and that higher salinity in the Bay attracted droves of saltwater oyster predators and disease, ultimately decimating the oyster population.
Georgia points to a more direct cause—Florida's mismanagement of its oyster fsheries. According to Georgia, Florida caused the collapse by overharvesting oysters and failing to replace harvested oyster shells. And even if low fows contributed at all, Georgia says, they were driven by climatic changes and other factors, not its upstream consumption. Of course, the precise causes of the Bay's oyster collapse remain a subject of ongoing scientifc debate. As judges, we lack the expertise to settle that debate and do not purport to do so here. Our more limited task is to evaluate the parties' arguments in light of the record evidence and Florida's heavy burden of proof. And on this record, we agree with the Special Master that Florida has failed to carry its burden. Page Proof Pending Publication Florida's own documents and witnesses reveal that Florida allowed unprecedented levels of oyster harvesting in the years before the collapse. In 2011 and 2012, oyster harvests from the Bay were larger than in any other year on record. Fla. Exh. 839; 4 Trial Tr. 956; 6 Trial Tr. 1391. That was in part because Florida loosened various harvesting restrictions out of fear—ultimately unrealized—that the Deepwater Horizon oil spill would contaminate its oyster fsheries. 3 Trial Tr. 767–769. A former Florida offcial, one of Florida's lead witnesses, acknowledged that these management practices “ `bent' ” Florida's fsheries “ `until [they] broke.' ” Ga. Exh. 1357, p. 1; 4 Trial Tr. 877.
The record also shows that Florida failed to adequately reshell its oyster bars. Reshelling is a century-old oyster- management practice that involves replacing harvested oyster shells with clean shells, which can serve as habitat for young oysters. Id., at 907–908; 17 Trial Tr. 4390. Yet in the years before the collapse, while Florida was harvesting oysters at a record pace, it was simultaneously reshelling its oyster bars at a historically low rate. See Direct Testimony of Romuald N. Lipcius ¶¶138–151 (Lipcius), and Demos. 15, 16; see also Ga. Exh. 568, p. 5 (recommending that Florida reshell 200 acres per year); 7 Trial Tr. 1692 (Florida reshelled 180 total acres in the 10 years before the collapse).
Georgia's marine ecologist, Dr. Lipcius, demonstrated the stark effects of Florida's increased harvesting and lax re- shelling efforts. Analyzing data on oyster densities precollapse and postcollapse, Dr. Lipcius found that mean densities in the Bay's most heavily harvested oyster bars dropped by an average of 78%, while mean densities increased by 3% to 13% at bars that either were not heavily harvested or had been reshelled. Lipcius ¶¶41–44. Dr. Lipcius also found negligible differences in salinity among the bars that he analyzed, suggesting that increased salinity did not explain the variance in oyster densities. Id., ¶¶48–51.
Florida does not meaningfully rebut this evidence.
Yet Florida nonetheless argues that Georgia's overconsumpPage Proof Pending Publication Page Proof Pending Publication tion—and the consequent increased salinity and predation— was the sole cause of the collapse, or at least a substantial factor contributing to it.* But here again, Florida's own witnesses suggest otherwise.
Dr. White, one of Florida's ecology experts, modeled how oyster biomass would have changed at two of the Bay's major oyster bars if Georgia had consumed less water in the years leading up to the collapse. His modeling showed that reducing Georgia's consumption by an amount “similar to the relief that Florida is requesting” in this case would have increased oyster biomass by less than 1.5% in 2012. Updated Pre- Filed Direct Testimony (PFDT) of J. Wilson White 49–51, fgs. 14, 15.
Florida does not explain how such minor fuctuations in oyster biomass could have averted the collapse. Instead, Florida points to testimony that increased streamfow would have had “larger” effects on oyster biomass at oyster bars closer to the river's mouth. 7 Trial Tr. 1725; see also id., at 1868–1870. But it was Florida's burden to quantify how much larger the effects would have been, and its experts did not model biomass changes at bars near the river. See 6 Trial Tr. 1571.
Other Florida experts reinforced Dr. White's biomass fndings. One expert found that salinity reductions of greater than 10 parts per thousand are “required” in order to reduce predation by rock snails—one of the oyster's fercest predators. Fla. Exh. 797, p. 38; Updated PFDT of Mark Berrigan ¶¶42–43 (Berrigan). Yet according to another Florida expert, salinity throughout the Bay would have declined by substantially less than 10 parts per thousand in 2012 even if Georgia had eliminated all of its consumption from the Basin. PFDT of Marcia Greenblatt ¶¶4, 27, 30. Together, these fndings further undermine the asserted *We have not specifed the causation standard applicable in equitable- apportionment cases. We need not do so here, for Florida has failed to establish a suffcient causal connection under any of the parties' proposed standards.
link between Georgia's consumption and decreased oyster biomass.
In response to this empirical evidence, Florida relies primarily on: (1) testimony from a local oysterman and a former Florida offcial that they witnessed high salinity and signifcant oyster predation, including at private oyster bars not subject to overharvesting, PFDT of Thomas L. Ward ¶¶33– 37; Berrigan ¶¶44–48; (2) reports by its own agency blaming the collapse in part on salinity and predation, Joint Exhs. 50, 77; (3) a fshery-disaster declaration by the National Oceanic and Atmospheric Administration (NOAA) adopting the conclusion of Florida's agency, Fla. Exh. 413, p. 3; and (4) feld experiments conducted by one of Florida's experts in the years after the collapse, which purport to demonstrate a link between increased salinity and predation, Updated PFDT of David Kimbro ¶¶63–90.
The fundamental problem with this evidence—a problem that pervades Florida's submission in this case—is that it establishes at most that increased salinity and predation contributed to the collapse, not that Georgia's overconsumption caused the increased salinity and predation. None of these witnesses or reports point to Georgia's overconsumption as a signifcant cause of the high salinity and predation. The NOAA, in fact, primarily blamed “prolonged drought conditions” and the Corps' reservoirs operations—not Georgia's consumption during drought conditions—for the elevated levels of salinity and predation in the Bay. Fla. Exh. 413, pp. 3–4. Other record evidence, moreover, indicates that the unprecedented series of multiyear droughts, as well as changes in seasonal rainfall patterns, may have played a signifcant role. See PFDT of Dennis Lettenmaier 17, fg. 8; Direct Testimony of Wei Zeng ¶¶144–152. Given these confounding factors, we do not think that Florida's evidence of high salinity and predation overcomes the data and modeling of its own experts, which show that Georgia's consumption had little to no impact on the Bay's oyster population. Page Proof Pending Publication Considering the record as a whole, Florida has not shown that it is “highly probable” that Georgia's alleged overconsumption played more than a trivial role in the collapse of Florida's oyster fsheries. See Colorado II, 467 U. S., at 316. Florida therefore has failed to carry its burden of proving causation by clear and convincing evidence.
B
Florida also argues that Georgia's overconsumption has harmed river wildlife and plant life by disconnecting tributaries, swamps, and sloughs from the Apalachicola River, thereby drying out important habitats for river species. The Special Master found “a complete lack of evidence” that any river species suffered serious injury from Georgia's alleged overconsumption, and we agree. Second Report of Special Master 22.
In seeking to prove injury, Florida relied primarily on species-specifc “harm metrics” developed by Dr. Allan, one of its ecology experts. Dr. Allan established minimum river-fow regimes that he believed necessary for certain species of fsh, mussels, and trees to avoid “signifcant harm” during dry months. Updated PFDT of J. David Allan ¶¶33– 61 (Allan). He then sought to quantify the harm to each species by totaling the number of days in which river fows fell below his thresholds. Id., ¶¶62–63.
What Dr. Allan did not do, however, is show that his harm metrics did or likely would translate into real-world harm to the species that he studied. Indeed, Dr. Allan provided no data showing that the overall population of any river species has declined in recent years. See 2 Trial Tr. 389–392, 395– 396. And other evidence casts signifcant doubt on Dr. Al- lan's harm metrics. For instance, the U. S. Fish & Wildlife Service found that the population of the fat threeridge mussel—one of the species Dr. Allan analyzed—“appears stable and may be increasing in size.” Joint Exh. 168, p. 125; Allan ¶42.
Page Proof Pending Publication Without stronger evidence of actual past or threatened harm to species in the Apalachicola River, we cannot fnd it “highly probable” that these species have suffered serious injury, let alone as a result of any overconsumption by Georgia. See Colorado II, 467 U. S., at 316.
* * * In short, Florida has not met the exacting standard necessary to warrant the exercise of this Court's extraordinary authority to control the conduct of a coequal sovereign. We emphasize that Georgia has an obligation to make reasonable use of Basin waters in order to help conserve that increasingly scarce resource. But in light of the record before us, we must overrule Florida's exceptions to the Special Master's Report and dismiss the case.
It is so ordered.
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