The Congo River estuary is characterised by a deep canyon that connects the river to the deep ocean by cutting through the continental shelf. Its estuary is influenced by high river discharge and micro-tidal conditions, with a large depth and limited vertical mixing. This restricts the supply of oxygen from the surface waters to the more saline bottom waters, leading to hypoxic and anoxic zones. We study the dynamics of the Congo River estuary by applying the multi-scale baroclinic coastal ocean model SLIM 3D (www.slim-ocean.be) to this topographically challenging environment. By allowing a high degree of flexibility in the representation of both the complex geometry and the strong stratification, SLIM 3D is able to simulate riverine, tidal and gravitational processes that drive the estuarine circulation. Model results compare favourably with in-situ data in the estuary, suggesting that the exchange flow is correctly simulated. The latter is characterised by a two-layer dynamics. The combination of the large river discharge, the strong stratification and the large depth results in a moderate freshwater Froude number and a very small mixing number. It makes the Congo River an outlier in state-of-the-art estuarine classifications, closer to fjords than salt wedge estuaries. Furthermore, using the age as a diagnosis sheds light on the spatial variability of the estuarine waters ventilation. Local maximum of renewing water age located just below the pycnocline is exceeded by old dense oceanic waters which stagnate at the bottom of the canyon for more than two months due to the small vertical mixing. It helps explain the hypoxic and anoxic conditions observed at the bottom of the submarine canyon.

刚果河河口的特点是一条深峡谷，穿过大陆架将河流与深海相连。它的河口受河流高流量和微潮条件的影响，深度大，垂直混合有限。这限制了氧气从地表水到更咸的底部水的供应，导致缺氧和缺氧区。我们通过将多尺度斜压沿海海洋模型 SLIM 3D (www.slim-ocean.be) 应用于这个地形具有挑战性的环境来研究刚果河河口的动态。SLIM 3D 允许高度灵活地表示复杂的几何形状和强烈的分层，能够模拟驱动河口环流的河流、潮汐和重力过程。模型结果与河口的原位数据相比具有优势，表明交换流被正确模拟。后者的特点是两层动态。河流流量大、分层强、深度大，淡水弗劳德数适中，混合数很小。它使刚果河成为最先进的河口分类中的一个异常值，比盐楔河口更接近峡湾。此外，使用年龄作为诊断揭示了河口水域通风的空间变异性。由于小的垂直混合，在峡谷底部滞留了两个多月的古老而密集的海洋水超过了位于紧斜线下方的局部更新水年龄的最大值。