天津近代開埠後，天津海關（洋關：下稱海關）設於紫竹林，它雖以關稅徵收為職責和目標，但也頗為重視海河下游水文資訊的調查和監測，因其為船隻安全通航的基礎，實際上也攸關稅收徵收，甚至影響天津近代城市發展。本文探討海關在1861-1899年間的不同階段，透過哪些方式和管道，蒐集和構建海河下游水文知識？接著，在水文認知基礎上，海關又是如何利用或應對於兩個重要面向？一是它如何逐步建立海河導航設備和措施，其次則是1880年代以來海河下游河道日漸淤淺和氾濫，嚴重影響商貿和民生，中外官員和商人群體均表關切，海關在河道疏濬和改善過程中，又扮演何種角色？文中嘗試藉由上述水文議題，更進一步了解海關的職責和工作意義，以及西方水文知識和技術發揮的作用。

After the opening of the port of Tianjin 天津 in the 1860s, the Tianjin Customs Office was established in Zizhulin 紫竹林. Although its stated objective was the collection of customs duties, it also attached great importance to the investigation and monitoring of the hydrographic information of the lower reach of the Hai River 海河, as it was the basis for the safe navigation of vessels, which also affected the collection of tariffs and even the modern urban development of Tianjin. This article examines the channels through which Tianjin Customs collected and constructed hydrological knowledge of the lower reach of the Hai River between 1861 and 1899. It then investigates how, on the basis of this hydrographic knowledge, Tianjin Customs made use of and responded to two important facets related to the river. First, it describes how Tianjin Customs sought to establish navigation equipment and measures. Second, from the 1880s the silting and overflowing of the lower course of the Hai River had seriously affected trade and the people’s livelihoods, causing Chinese and foreign officials, as well as merchant groups, to express concern. The article looks at the role Tianjin Customs played in the process of dredging and improving the watercourse. In sum, this article uses the above-mentioned hydrological issues to gain a better understanding of the significance of Customs in the late Qing dynasty and the role played by Western hydrological knowledge and technology.