Bacteria move cooperatively for many reasons: to invade host tissues, to seek out nutrients, or to erect structures for spore spreading. Swarming is a type of cooperative surface-associated motility, in which each cell is self-propelled by its own flagella. Twitching and social gliding also depend on a motor driven cellular appendage; type IV pili are extended toward a surface, adhered, then retracted, pulling the individual cell forward toward the adhesion. Intriguingly, many types of collective motility do not require the use of known motor-driven appendages; these include gliding, adventurous gliding, sliding, and spreading. The physical mechanisms underlying these kinds of cooperative motility are not yet clear, yet a clue comes from the physical chemistry of their excretions; all of these motor-independent types of motility involve amphiphilic molecules. Here we show that spatial concentration gradients of these amphiphilic molecules give rise to surface tension gradients that drive biofilm spreading. Moreover, we show that polymer production also drives bacterial biofilm spreading, and is triggered by a gradient of nutrient depletion within the colony. Thus, it is the spatially heterogeneous excretion of complex fluids by bacteria that gives rise to these two types of biofilm spreading force.