Some reports have also found that the Nam Ngum dam has adversely affected aquatic life downstream. Careful analysis has revealed that water released from the Nam Ngum reservoir has different quality as compared to the water in undammed natural rivers. Specifically the dissolved oxygen levels in the water released from the reservoir are unfavorable for aquatic life most of the year, especially during the wet season. Similarly some reporters have noted an unsafe level of hydrogen sulfide. The dam also actively blocks migration by upland fish into the downstream part of the river, which secludes these fish from active spawning sights. These deteriorations in the water quality and biodiversity of Nam Ngum river, pose a significant risk to the overall river ecosystem.