The Corps of Engineers has released maps that show where the floodwater released through the Morganza Spillway will goes, how long it will take to get there, and how deep it is expected to get. It will over time cover all of the Atchafalaya Basin between the guide levees and all of the Terrebonne Basin to the Bayou Lafourche ridge. The maps can be found here. If you need to review the structure of the floodway, go here.
In the first 48 hours water will fill the Morganza Floodway and the eastern half of the Atchafalaya Floodway between the East Protection Levee and the eastern Atachafalaya River Levee, which ends a little north of I-10. Within 30 hours it will come to the end of the western Atchafalaya River Levee and start filling the Atchafalaya Floodway between the river levee and the West Protection Levee. The water will move north through that portion of the floodway.
That water backing up outside the floodway at 144 hours and 192 hours comes from water spreading out through the western Terrebonne Basin south of the floodway.
At 72 hours the water will have filled the floodway, reached Morgan City, and begun streaming into Atchafalaya Bay along the Lower Atchafalaya River and the Wax Lake outlet. The Gulf Intracoastal Waterway crosses the Lower Atchafalaya, picks up water streaming out of the floodway, and carries east to the Vermillion Basin and west to the Terrebonne Basin at 84 hours, reaching the Bayou Lafourche ridge at 192 hours.
The Corps has also estimated how deep the water will get at each point along the way. Maps found at AskLouise.gov.