I'm not quite sure what point you're trying to make here, but the Addicks and Barker Reservoirs are detention basins that are indeed dry most of the time, but fill up with stormwater after heavy downpours. The accepted tagging scheme for detention basins is natural=water + water=basin + basin=detention [1]. If this doesn't look right to you, consider posting a message on the community forums [2].

[1] https://wiki.openstreetmap.org/wiki/Tag:basin%3Ddetention

[2] https://community.openstreetmap.org/