The area north and west of Basra is crisscrossed by causeways built up over square farm fields that are flooded and dried for planting. This area was much studied by the Pentagon as that was where most of the fighting took place during the 1980s Iran-Iraq war. Baghdad sits in a bend of the Tigris river. Lots of bridges to be crossed if you want to take the city. But you can come at Baghdad from many directions and the Iraqis have to expect an American army to approach from an unexpected direction. Some of the bridges many not be bombed, and plans laid to take them intact using commandos.
The U.S. Army has six bridging companies, units that can quickly build bridges (for M-1 tanks and all other vehicles) over wide rivers like those found in Iraq. Three of these companies are in the reserves and have been mobilized. Two of the companies are in the regular army and stationed in the United States. These are on their way to the Persian Gulf. So is the sixth company, a regular army unit stationed in Germany. Each combat division has an engineer battalion with some bridging capability, but not enough to handle wide rivers. While engineers were busy during the 1991 Gulf War (some types of sand are harder to get across), they did not have to deal with all the "wet work" that will be encountered when you enter the Tigris-Euphrates river region.
In central and southern Iraq, there are lots of water obstacles. You have rivers, irrigation canals, mudflats and fields that can be flooded. There's a lot of soft ground in general. The Tigres-Euphrates "valley" has a high water table and a lot of soft ground. The Iraqis keep their armored vehicles on the roads and known hard ground, but American troops tend to be more adventurous. That's one thing the Special Forces scout teams are no doubt checking out. Probably Navy SEALs as well (they train to go in and grab soil samples to see which vehicles might have trouble moving through.)