Hospitals across the nation are being swept up in the biggest wave of mergers since the 1990s, a development that is creating giant hospital systems that could one day dominate American health care and drive up costs. The consolidations are being driven by a confluence of powerful forces, not least of which is President Obama's signature health care law, the Affordable Care Act. That law, many experts say, is transforming the economics of health care and pushing a growing number of hospitals into the arms of suitors. The changes are unfolding with remarkable speed. Two big for-profit hospital chains, Community Health Systems of Tennessee and Health Management Associates of Florida, are combining in a $7.6 billion deal.