*2.3. Environmental Flows*

Transformation of the flow regime downstream from the dam is among the most adverse impacts of dam construction on freshwater ecosystems. One of the compensatory measures to mitigate the impacts of dam construction is the implementation of environmental flow, which is the release of a specific volume of water from the reservoir to mimic the characteristics of the natural flow variability of water and sediments [7].

There are about 200 methods for determining environmental flows [55] being used globally. Regardless of the chosen method, it is important to characterize the flow regime and its intra- and interannual variability in natural conditions. These characteristics describe the water regime to which the ecosystem has been adapted.

In Russia, guidelines for the development of reservoir operating rules indicate that reservoirs should also be used for environmental flow releases [56]. The method prescribed in "Methodological Guidelines for Developing Standards to Assess Environmental Flow" is based on preserving the freshwater ecosystem in a state where its restoration potential is not disturbed by determining the critical flow volume [57]. Environmental flow is calculated based on the difference between the values of annual runoff and the volume of allowable water withdrawal. In this study, the applicability of various methods to freshwater ecosystems of the Amur basin was assessed, but we did not seek to design another specific methodology.

#### **3. Results**

The description of the results is characterized, to the extent possible, both from technical outcomes of the assessments as well as from the public participation and policy processes in which they were used.

#### *3.1. Basin-Wide Hydropower Assessment*

#### 3.1.1. Main Assessment Findings

Using the basin-wide rapid assessment tool, how potential impacts depend on the degree and pattern of hydropower development was analyzed, with the following results.

At initial phases of development in unaffected basins, individual hydropower dams with similar production may have a more than 10-fold difference in potential basin-wide environmental impacts (Figure 3).

**Figure 3.** Impact of individual hydropower dams.

It was found that as the development of hydropower potential progresses, the difference between "best" and "worst" development options decreases. The scenarios selected for analysis in this paper are described in Table 1.



Figure 4 shows the scenarios with minimum and maximum impact, which were identified from all scenarios utilizing 10% of economic hydropower potential. In terms of the integral impact index, they differed six-fold, still showing ample potential for avoiding the most negative basin-wide impacts.

Figure 4 also shows an analysis of the actual development in the Amur basin (Scenario 25% actual), which was just below 25% of total economic hydropower potential, indicating that the same electricity production could have been achieved with a much smaller integral impact (Scenario 25% min), while the degree of impact in a scenario with maximum possible damage (Scenario 25% max) was 100% more than in Scenario 25% min, but only 10% more than in Scenario 25% actual.

**Figure 4.** Scenario analysis and the corresponding impact due to different levels of development of the hydropower potential in the Amur basin.

**Figure 5.** Relationship between the impact factors in scenarios when dams are located in the upper (Up) and lower (Down) parts of the Amur basin.

**Figure 6.** Potential scenarios of hydropower development based on the current situation.

Further modeling of scenarios with the development of 75% of technical potential resulting in maximum (Scenario 75% max) and minimal (Scenario 75% min) impact demonstrated that few choices remain between "worse" and the "worst" impacts, with only a 15% difference between the most damaging and least damaging options. Further development of "full economic potential" (Scenario 100%) led to only a 25% increase in integral impacts.

This shows that the common argument by hydropower proponents that "only 10% of hydropower potential is developed and therefore we need to develop much more" likely has an acute conflict with the notion of sustainable development because a greater proportion of basin-wide hydropower development leaves no room for "low-impact" sustainable development scenarios.

On the other hand, most impact has usually been achieved at relatively low levels of hydropower development (at least in the case of the Amur), while the additional development of dams in an already heavily affected basin brings significantly smaller additional incremental impacts. This also calls for the cautionary use of "efficiency" measures relating impact to the production of electricity, because it does not caution against excessive development in a given basin.

Dam location seems to be the leading factor defining the integral impact of hydropower development scenarios. According to this assessment methodology, "dams located downstream" are the dams that block a greater part of the overall basin. To test how this factor influences integral impact, dams were grouped in 8–9 scenarios, each having a total production of 5–6 GWh/year, with projects in a given scenario having similar "blocked basin" index values (see Figure 5). Some scenarios consisted of a single large dam on the main stem. Integral impact per unit production (*INT3*/kWh) increased for scenarios with dams located further downstream, primarily due to the increased share of the blocked basin (*IMP\_block*, %), while the variation of the other two indices had no clear trend as we moved downstream. For "downstream" dams, the value of *INT3*/kWh was usually 2–2.5 times higher than for scenarios with "upstream" dams. If other aspects of fragmentation were included in the equation, dam location became an even more influential factor in defining the overall impact.

For the Amur basin, this assessment concludes that minimal additional impact is associated with the development of hydropower cascades on tributaries that already have dams. This minimizes fragmentation of the basin, interference with sediment flows, and other negative influences. Conversely, the development of dam cascades on the main stem of the Amur will lead to maximum negative impacts. The addition of even one relatively small dam (Khingansky—Taipinggou) on the main channel will lead to a sharp increase in the cumulative impacts of hydropower in the basin (see Figure 6).

However, "sustainable" development of hydropower in the Amur basin is significantly limited by the degree of cumulative impact from existing dams, which is already very close to the maximum possible impact that could be induced by any scenario with comparable electricity production.
