A Travis County state district judge on Friday tossed a state law that would force the city of Houston to sell its water rights in a proposed reservoir west of Simonton.

The law, which breezed through the Legislature last session and was signed into law by Gov. Greg Abbott, requires the city to sell its rights in the proposed Allens Creek Reservoir by the end of the year for up to $23 million.

The city sued the state and the Brazos River Authority in July, claiming the law was unconstitutional in part because it violates prohibitions on retroactive laws and on forced sales of municipal property that have a public use.

In a ruling issued Friday evening, state district Judge Karin Crump of the 250th District Court agreed with the city’s interpretation, finding the law violates several provisions of the Texas Constitution and Local Government Code.

Crump ruled that the law constitutes a forced sale and “changes the legal consequences of acts completed before the bill’s effective date without sufficient findings to justify passage” of the bill.

Mayor Sylvester Turner, who fought against the bill, has argued that the payment offered in the law would reimburse only what Houston has spent on the project, not compensate the city for the loss of 15 percent of its surface water rights, nor provide it a replacement water source.

The law’s backers argue Houston has developed its water rights in the Trinity and San Jacinto river basins so thoroughly that it has no urgent need and no immediate plan to build the reservoir, whereas huge petrochemical plants along the Lower Brazos have skipped expansions due to a lack of water resources.

Reporter Mike Morris contributed to this report.

jasper.scherer@chron.com