Venice in mid to late March, risk floods?

Venice can "flood" any time of the year, but more frequently in the winter. It's common enough that the city has a system of alternative routes and raised walkways. It's also reasonably predictable because it occurs with the tides. Apart from suggesting you take along waders (which would be ridiculous) I can't think of more to tell you. "Aqua Alta" happens.

Elizabeth, I have just returned from Venice. Winter produces more acqua alta than many other months. I experienced none of this from Dec 19-23. I did see that the island was much prepared for this event. There were frames and boards in the center of walkways waiting to be set up if the tide came in high. In addition, many bridges where already set up with ramps over the steps. Worst case, you may have to purchase yellow high boot covers to protect you from this type of event. I would recommend not to worry and plan your trip and have fun.

We spent 5 nights in Venice in mid March 2007. No floods, and the weather was perfect!

It has flodded in March before...but...usually it's only St. Marks that's the huge problem anyway (not the whole city!) and they put up walkways with boards...and you can buy rain boots in a shop it you reallly need them :-) I wouldn't worry too much ab out it or let it stop you. Persoanlly, I'd rather flood waters than flodds of tourists ;-)