The rain has, at least for the moment, slowed down in Houston this morning. But that doesn’t mean that it’s stopped and now a new problem is threatening the city in a major way. We’ll be keeping our eyes on the two primary safeguards against a catastrophic flood in the downtown area, the Addicks and Barker reservoirs. As of around 6 o’clock this morning local time, the Addicks reservoir was within a few inches of going over the top of the dam wall and still rising. (You can track the level of the water in that reservoir on the county emergency management page here. It’s updated every 8 minutes.) There are two emergency spillways, but the rest of it is just an earthen structure built after World War II and it wasn’t supposed to ever get to this level. Even opening the gates on the dam hasn’t stopped the rise of the water and authorities are predicting that it will overflow today.

UPDATE: (Jazz) As of 7:45 local time the Addicks dam had topped and water was flowing over the rim. Officials announced that they are opening the gates wider, so a lot of water is heading into the city. They further said that controlled releases will be taking place “for months” after the storm is over. This recovery is going to be going well into 2018.

That’s bad news for all the neighborhoods downstream from there and according to Fox News they have not been completely evacuated. The worst case scenario (and we should stress that there are no signs of this yet) is if water cresting the top begins to erode the earthen walls. There are no words to describe what would happen to a major portion of Houston if the dam failed.

Yesterday we talked about the coming blame game in terms of why Houston wasn’t evacuated sooner, but this situation with the dams dates back much further. NBC meteorologist Bill Karins was on Twitter this morning describing it as a failure of planning and pointing to an excellent article at Slate where they describe how unchecked urban development allowed so many structures to be built in places which are now at extreme risk.

[A]s development has sprawled west along the Katy Freeway, more and more water is being funneled into the region’s creeks, filling the reservoirs faster. “Of the 10 largest pools that have accumulated in the reservoirs, nine have occurred since 1990 and six of those were since 2000,” ProPublica wrote last year.

Meanwhile, developers swooped in and built tract houses up to the very brink of the reservoirs, which appear in dry times to be forests. It’s probably a very pleasant place to live, except when it isn’t. During last year’s Tax Day floods, those subdivisions on the reservoirs’ western edges flooded. Now they are flooding again. None are in the 100-year floodplain. Most are in the 500-year floodplain, areas that FEMA predicts will flood once every 500 years. They are not obligated to have flood insurance. They have flooded two years in a row.

Those homes should probably never have been built. Now they’ll be flooded for quite some time: “Homes upstream will be impacted for an extended period of time while water is released from the reservoirs,” the Corps wrote in a press release. The reservoirs will take between one to three months to drain.

On the one hand, you can almost have some sympathy for the people who decided to develop all of that property. It wasn’t supposed to flood more than once in 500 years. But, as the Slate authors said, it’s a very pleasant place to live, except when it isn’t. Somebody was in charge of emergency planning over all of these decades and they needed to know that this day would come eventually.

The most immediate need at the moment is to ensure that everyone still in their homes downstream from the Addicks reservoir is gotten out of there. (These are some of the people who were told by municipal officials to “shelter in place.”) There will be time for finger pointing later. But when everyone is (hopefully) safe and the city begins to dry out, it looks like Houston will be facing the same tough questions that New Orleans was after Katrina. The city’s layout needs to be reexamined with an eye toward future major storm events. That may mean abandoning construction in some of the lower elevation areas and focusing on urban expansion upstream. Because if this is happening now, it’s obviously going to happen again. It’s a question of when, not if.