I would Have to say both Dyiana's and Morethan1's answers are good and close to the point. The depression was more that just a down financial market. Many portions of our farm land was being destoryed by weather and by poor land management. Include high transportation costs or to so some areas lack of transportation. Much of our food problems were in the Midwest Section (the bread basket) of the United States and the east cost. In order to feed the masses we began to set up food distribution centers. Much like today in some Middle Eastern countries. In Haiti it is only worse, one of their basics have become (no humor intended) mud pies made with dirt, water, cooking oil, and possibly corn or corn meal.
Once again though a part of the problem is the cost of transportation. Also to increase profits more want to sell on the International Markets, or shall be say greed. We could also increase so many of the one use only grains developed by Dow Chemical, and Monsanto and their likes. Where at one time we used a portion of the havested grain for the nest season. With and of the current crops growns we no longer have that option. New grain must be purchased every year.
Using our religious texts, the Bible, during the good years we were to save the grain so that every 7th year the land could go fallow and nothing would be grown. But so few Americans as well as other countries grow every year and refuse to save for this event and other weather related events.
As we ignore God, we ignore more of his teachings, we strive to conitually over produce in every aspect of our economy.
And what have been the results of ignoring God's Laws? Well we haven't save enought food to feed the world, America used to be the worlds leading producer, With high energy we can't transport what we can grow, and because grains and fertilizers or one longer natural things we can't afford to buy what we need to even feed ourselves. Basically turning over more land and more water rights to Big Agra Business. And with out enought to eat and enough clean water we bring back the disease and pestilences thought to be eradicatied and devolope new ones we can't control.
A quick look at Zimbowie (sp?) . Once considered the bread basket of Africa, after taking the farms away from those who knew how to run the big argra farms, then going through a period cleansing, they have nothing, no food, no clean water, and a population being eaten alive by the plague. Their government denies the disease, instead allowing and forcing the sick to leave the country to infect more and more, and all the while people (the United Nations, the United States, the European Union and the African Union) sit back and do nothing.
I hope this has helped answer your question concerning the causes and the possible results of bread lines. As for now, our only hope is GOD, the Almighty.