concurring in part and dissenting in part:

I concur in the majority opinion, except with respect to Part IV C. I agree that the BLM did not sufficiently consider the potential cumulative impacts posed by the four anticipated timber sales. I do not agree, however, with the majority’s decision not to require the BLM to produce a single NEPA review document at this time. Although federal agencies have considerable discretion to define the scope of NEPA review, “[a] single NEPA review document is required for distinct projects when there is a single proposal governing the projects, or when the projects are ‘connected,’ ‘cumulative’ or ‘similar’ actions under the regulations implementing NEPA.” Native Ecosystems Council v. Dombeck, 304 F.3d 886, 893-94 (9th Cir.2002) (internal citations omitted). In this case, there is both a single proposal governing the four anticipated projects and the projects constitute cumulative actions under the implementing regulations. Therefore, I would require a single NEPA analysis. 1

I. A Single Proposal Governs the Timber Sales

In early 1999, the BLM created a plan to actively manage the South Fork Little Butte Creek (“SFLBC”) watershed under a single silvicultural prescription, titled “SFLBC Project Timbers Sales (FY 2000-2003).” The proposal described the forest management objectives and methods for harvesting and maintaining the SFLBC portion of the watershed. After completing 85% of the environmental review on *1002 the project as a whole, the BLM found that it was unable “to complete the protocol for Survey and Managed (mullosk) species” as scheduled. In order to avoid delaying the proposed sales while it finished the protocol, the agency decided to split the environmental review of the project into multiple parts. Significantly, even after formally splitting the project into four parts, BLM staff continued to treat the four areas together as part of a single watershed management project, maintaining a “project map” of the four areas and discussing the projects jointly as the “Little Butte Creek” project.

Because the project was conceived as a single project and continued to be discussed and planned as a single project even after division into four parts, I would hold that “a single proposal governfed] the projects,” and therefore would require a single NEPA review document. Cf. Earth Island Inst. v. U.S. Forest Serv., 351 F.3d 1291 , 1305 (9th Cir.2003) (“In this case, because there is nó comprehensive plan covering both forests, Plaintiffs may only prevail by showing that the separate actions are ‘connected, cumulative or similar’ .... ”); Native Ecosystems, 304 F.3d at 894 (holding that a single NEPA review document was not necessary in part because “there [wa]s no Gallatin Il-wide proposal to amend road density standards”).

II. The Four Timber Sales Qualify as “Cumulative Actions”

A single NEPA analysis is also required when several actions qualify as “cumulative actions,” under the regulations — that is, when the actions will have “cumulatively significant impacts.” 40 C.F.R. § 1508.25 (a)(2); see also Earth Island, 351 F.3d at 1305-06; Thomas v. Peterson, 753 F.2d 754, 758 (9th Cir.1985). This requirement exists in order “to prevent an agency from ‘dividing a project into multiple ‘actions,’ each of which individually has an insignificant environmental impact, but which collectively have a substantial impact.’ ” Earth Island, 351 F.3d at 1305 (citing Thomas, 753 F.2d at 758 ). Significantly, the regulations recognize that “[cjumulative impacts can result from individually minor but collectively significant actions taking place over a period of time.” 40 C.F.R. § 1508.7 .

Of course, it is not possible to know definitively whether agency actions will have cumulatively significant impacts before an environmental assessment is completed because the very purpose of an EA is to determine what effect a project will have on the environment. As the majority recognizes, see op. at 15280, our circuit has addressed this problem by requiring that a single analysis be performed when the record “raise[s] substantial questions that [the proposed agency actions] will result in significant environmental impacts.” Blue Mountains Biodiversity Project v. Blackwood, 161 F.3d 1208, 1215 (9th Cir.1998); see also Sierra Club v. U.S. Forest Serv., 843 F.2d 1190 , 1194-95 (9th Cir.Í988); Thomas, 753 F.2d at 759-61 .

For example, in Blue Mountains, we required a single analysis even though the Forest Service had attempted to assess five distinct timber sales separately. The sales at issue were located in one watershed, were part of a “coordinated” forest management project, were reasonably foreseeable, and an estimated time line was established before the first EA was complete. Id. at 1214-15. Although the Forest Service’s cumulative impact analysis was flawed-making it impossible to know definitively the extent of the potential cumulative impact — we concluded that “[a]t the very least, these sales raise[d] substantial questions that they [would] result in significant environmental impacts.” Blue Mountains, 161 F.3d at 1215 . As a result, we held that a single NEPA document was necessary. Id.

*1003 As in Blue Mountains, the four sales at issue in this ease are located in one watershed, are part of a coordinated project, are reasonably foreseeable, and an estimated time line was established before the first EA was complete. See op. at 992, 1001. Also as in Blue Mountains, there is at least a substantial question as to whether the four sales will result in cumulatively significant environmental impacts. 2 See op. at 997. Both of the EAs recognize that each individual timber sale will increase the risk of higher magnitude flow events within the single watershed causing damage to soil and to the habitat of endangered salmon. The EAs also both recognize that each individual sale will adversely affect what the BLM itself identifies as a particularly crucial habitat for the threatened northern spotted owl. In fact, as the majority recognizes, 59% of the total suitable owl habitat within the critical link between the Cascades and the Klamath Mountains — a total of 1,881 acres — will be lost when just three of the projects are considered. Finally, the EAs state that each sale will be detrimental, at least in the short term, to water quality in the area — water quality that is already substandard under the Clean Water Act. Because individually the sales will have adverse impacts on soil, water quality, and the habitats of endangered salmon, the threatened northern spotted owl, and other special status species, 3 and because the sales are within a single watershed, there is, at the least, a substantial question as to whether the cumulative environmental harm will be significant. Therefore, I would hold that a single NEPA document is required. ■

In concluding otherwise, the majority relies on Native Ecosystems, 304 F.3d at 895 . However, Native Ecosystems is factually distinct. Unlike both Blue Mountains and the present case, the plaintiffs in Native Ecosystems were not challenging the environmental review of timber sales. Rather, they challenged the Forest Service’s site-specific decisions to waive the maximum road density rules on certain areas of Forest Service land to permit the construction of sufficient roads to proceed with timber harvest activities. Id. at 890-91 . The timber sales themselves were already approved in compliance with NEPA. Furthermore, in Native Ecosystems, unlike in both Blue Mountains and the present case, the road density amendments were not within a single watershed. Id. at 894 . Finally, in Native Ecosystems, unlike in this case and Blue Mountains, the challenged actions were not part of a single decision or plan by the agency.

The majority cites only one similarity between Native Ecosystems and this case: the incremental timing of the decisions at *1004 issue. But Native Ecosystems discussed timing only within its broader point that the challenged actions were not part of a single decision or plan. Furthermore, the NEPA regulations do not require “cumulative actions” to occur simultaneously, but instead define cumulative impacts as “collectively significant actions taking place over a period of time.” 40 C.F.R. § 1508.7 (emphasis added).

Indeed, to require agency actions to be simultaneous in order for them to fall within the definition of “cumulative actions,” would undermine the purpose of § 1508.25(a)(2). An agency could avoid a single NEPA analysis that fully considers a plan’s impact on the environment simply by breaking the project into phases. This is exactly what the regulations of § 1508.25(a)(2) were meant to avoid. See Earth Island, 351 F.3d at 1305; 40 C.F.R. § 1508.27 (b)(7)(“Signifieance cannot be avoided by ... breaking [an action] down into small component parts.”). 4

III. Conclusion

Because the four sales were governed by a single proposal and because there are “substantial questions” about the potential cumulative impacts posed by the sales, I would require the BLM to analyze the sales in single NEPA document.

. Because I conclude that the timber sales are "cumulative actions” under 40 C.F.R. § 1508.25 (a)(2), I do not reach the issue of whether the timber sales are "similar actions” under 40 C.F.R. § 1508.25 (a)(3).

. Without denying the similarities between this case and Blue Mountains, the majority dismisses Blue Mountains simply because it does "not specifically cite § 1508.25(a)(2) to support its conclusion.” Op. at 999. However, numerous Ninth Circuit cases cite Blue Mountains as authority when interpreting § 1508.25(a)(2). See, e.g., Earth Island, 351 F.3d. at 1305; Native Ecosystems, 304 F.3d at 895 ; Wetlands Action Network v. U.S. Army Corps of Eng’rs, 222 F.3d 1105 , 1119 (9th Cir.2000).

. Special status species are those species that are federally listed as endangered, threatened, proposed, or candidate, or that the Oregon State Office of BLM lists as sensitive or assessment species. Other special status species present in the SFLBC watershed that would be adversely affected by the timber sales include; long-legged myotis, fringed myotis, Yuma myotis, western bluebird, pi-leated woodpecker, great gray owl, western pond turtle, California mountain kingsnake, common kingsnake, bald eagle, northern goshawk, flammulated owl, northern saw — whet owl, Lewis’ woodpecker, western meadowlark, Townsend's big-eared bat, long-eared myotis, pacific pallid bat, and silver-haired bat.

. Given the factors that tend to show a substantial question as to cumulatively significant impacts and our conclusion that the agency erred significantly with respect to its environmental assessments, I would not afford dis-positive weight to the concept of deference. It is significant that, as the majority points out, less deference is afforded for "cumulative actions” than for "similar actions,” op. at 1001; see also Earth Island, 351 F.3d at 1306, and that a single NEPA analysis is mandatory under the regulations when the agency engages in cumulative actions. Op. at 1001.