Cities around the world are increasingly prone to unequal flood risk. In this paper, I “materialize” the political ecology of urban flood risk by casting stormwater drains—a key artifact implicated in flooding—as recombinant socionatural assemblages. I examine the production of flood risk in the city of Bangalore, India, focusing on the city's informal outskirts where wetlands and circulations of global capital intermingle. Staging a conversation between Marxian and Deleuzian positions, I argue, first, that the dialectics of “flow” and “fixity” are useful in historicizing the relational politics of storm drains from the colonial to the neoliberal era. Second, flood risk has been heightened in the contemporary moment because of an intensified alignment between the flow/fixity of capital and storm drains. Storm drains—and the larger wetlands that they traverse—possess a force‐giving materiality that fuels urban capitalism's risky “becoming‐being”. This argument raises the need for supplementing political‐economic critiques of the city with sociomaterialist understandings of capitalism and risk in the post‐colonial city. The paper concludes with reflections on how assemblage thinking opens up a more distributed notion of agency and a more relational urban political ecology.