Samuel Dennis Warren came to Westbrook in 1854 because it had something that he couldn’t build himself: a river to power his paper mill. He built the mill, and the mill built Westbrook.

Now a new generation of businesses will look at Westbrook and find that it has something that they could never build for themselves: a compact downtown with existing buildings and room for growth.

Like many former industrial cities, Westbrook is trying to reinvent itself at a time when small businesses and startups have taken the place of the large operations that once dominated the landscape. City leaders want to bring about a rebirth of their business district by removing the paper mill’s dam, and developing a commercial waterfront and aquatic recreation area on the banks of the Presumpscot.

They are building on their greatest asset, an existing downtown that could one day be home to a wide variety of employers providing as many jobs as the S.D. Warren mill in its heyday.

Maine has been slow to recognize that its many downtowns are as valuable a resource as its natural features. But they offer a constellation of advantages for small businesses that are hard to reproduce. A large employer can build a facility in a suburban office park with parking and a cafeteria. Some can even add a gym or a child care facility as perks to attract and keep high-quality employees. But a small business can’t do that.

In a downtown setting, however, they don’t have to. If the worksite is near restaurants, health clubs, day care centers and other services, the employer doesn’t have to provide them. A small downtown business can also share services with neighbors — for instance, all using the same computer consultant instead of staffing their own tech departments.

A developer can try to re-create that, but new construction is expensive and rents would be high. The advantage of an existing downtown is that its older buildings can offer more affordable options for businesses that are just getting their start.

The key to downtown success is diversity of uses that let different people use the same infrastructure at different times, sharing the costs. A city like Westbrook with an existing downtown has a great asset, and city officials are smart to try to build on it.