Hydropower development is accelerating in the tropics, especially construction of small dams. Small dams are often classified as those lower than 15 m in height or with reservoir capacity less than 3 × 10⁶ m³. Small dams frequently operate with water diversions from the main river channel, thereby affecting river connectivity through the creation of a dewatered reach. However, the extent and impact of dewatered reaches have been overlooked in most analyses of river fragmentation and dams to date. In Costa Rica, small dam development increased during the 1990s, ahead of current trends in other tropical regions. This study examined the effects of dams and their dewatered reaches on river connectivity at a national scale in Costa Rica. We adapted the Dendritic Connectivity Index and applied it to the four basins where over 80% of hydropower development has occurred. Our results showed that in two basins, the Sarapiqui and San Carlos, dewatered reaches contributed most to river fragmentation. In the Tarcoles and Reventazon basins, dams and dewatered reaches had similar effects on river fragmetnation. These different patterns can be explained by the spatial arrangement of dams in the individual river networks. We also found that dewatered reaches extended the elevation range of rivers affected by hydropower activity; this expansion was most significant in cases with interbasin water transfers. We discuss the implications of our findings on connectivity losses from dams for fishes whose range overlaps with hydropower activity. Our study may provide insights for other tropical countries with ongoing and future small dam development.

中文翻译：

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哥斯达黎加的大坝和河流脱水导致河流破碎化分析

热带地区的水电开发正在加速发展，特别是小型水坝的建设。小型水坝通常被归类为高度小于15 m或水库容量小于3×10 ⁶  m ^{3的那些。}。小型水坝经常通过从主要河道引水来运行，从而通过形成脱水段影响河流的连通性。但是，迄今为止，在大多数对河流破碎和大坝的分析中都忽略了脱水河段的范围和影响。在哥斯达黎加，1990年代小型水坝开发增加，超过了其他热带地区的当前趋势。这项研究在哥斯达黎加的全国范围内研究了水坝及其脱水段对河流连通性的影响。我们对“树突连通性指数”进行了调整，并将其应用于四个水电开发量超过80％的盆地。我们的研究结果表明，在萨拉皮基和圣卡洛斯两个盆地中，脱水河段对河流破碎的影响最大。在Tar​​coles和Reventazon盆地，水坝和脱水河段对河流脆弱化的影响相似。这些不同的模式可以通过单个河网中大坝的空间布置来解释。我们还发现，脱水河段扩大了受水电活动影响的河流的海拔范围；在跨流域调水的情况下，这种扩展最为显着。我们讨论了研究结果对水坝活动范围重叠的鱼类的水坝连通性损失的影响。我们的研究可能会为正在进行和未来的小水坝开发的其他热带国家提供见识。我们还发现，脱水河段扩大了受水电活动影响的河流的海拔范围；在跨流域调水的情况下，这种扩展最为显着。我们讨论了研究结果对水坝活动范围重叠的鱼类的水坝连通性损失的影响。我们的研究可能会为正在进行和未来的小水坝开发的其他热带国家提供见识。我们还发现，脱水河段扩大了受水电活动影响的河流的海拔范围；在跨流域调水的情况下，这种扩展最为显着。我们讨论了研究结果对水坝活动范围重叠的鱼类的水坝连通性损失的影响。我们的研究可能会为正在进行和未来的小水坝开发的其他热带国家提供见识。