Three summers ago, David Heicher traveled through the Susquehanna watershed to take a look at the conditions of its rivers and streams during dry conditions.

He found people near access sites dragging their boats around, trying to find water deep enough to float in. He found streamside wetlands that had become totally disconnected from the receding waterways. He found silt and algae building up in areas where fast-flowing water normally scoured the rocky bottom clean.

And in some places, he found no stream at all. “There were some locations in New York state where the only water that was left in the stream was pools,” said Heicher, chief of the Susquehanna River Basin Commission’s water quality and monitoring program. “Some small streams were totally dry.”

The 1995 drought was not, by historical standards, unusually severe. In fact, in many places water levels did not hit the seven-day, 10-year low flow mark used as a benchmark for wastewater treatment plant design — the level streams are expected to drop to for at least seven days once a decade.

Yet Heicher found evidence of impacts almost every place he looked, especially for stream-dwelling species. And that, he said, raises concerns that more frequent or severe dry spells could take a biological toll throughout the watershed’s streams.

“If you have a stream that’s going to dry up once every 10 years, there is some time in between for recovery to take place,” Heicher said. “If, as a result of increased consumptive uses, it’s going to dry up once every five years, you can expect to see some differences biologically.”

It’s not just a problem that affects headwater species such as trout. Migratory species of concern in the Bay, such as shad, could suffer ill-effects of more frequent, prolonged and severe droughts, he said.

Low flows can influence the migration patterns of such species, and in some cases, migration routes are blocked by obstacles exposed by low water that fish normally would swim over.

Heicher combined his observations from his 1995 Susquehanna basin trip with a review of scientific literature on low-flow effects on streams to produce a report on the issue. His report is a lengthy summary of impacts.

Reduced flows, for example, can magnify the impact of pollutants by reducing the dilution capacity of the stream. During the 1995 drought, Heicher noted, nutrient concentrations were elevated in many areas with low flows, resulting in the formation of dense algae mats that covered rocks.

When streams have low levels of water, they are more susceptible to temperature increases because there is less water to heat, and because the stream tends to recede from the bank — which is often shaded — toward the mid-channel which is more likely to be exposed to sunlight.

Warmer temperatures cause a reduction in the amount of dissolved oxygen in the water which, in turn, can limit the growth, reproduction and survival of fish and other aquatic life. The amount of dissolved oxygen is further reduced because low water levels reduce the effect of riffles in the stream channel that would help to replenish oxygen.

All that helps to set up deadly conditions. “The potential for fish kills is generally increased during droughts.” the report said.

Fish can also find themselves stranded in pools or channels that are cut off from the main stream and gradually dry up. Fish crowded into smaller habitats become more vulnerable to disease and become easy pickings for predators such as birds, raccoons, other fish — and anglers.

As the stream channel narrows, the amount of habitat for fish and other organisms, such as insects that serve as fish food, can be greatly reduced. Insects occupying certain habitats, such as riffles, tend to be impacted more severely than others, the report noted.

Reduced current velocity can cause behavioral changes in some fish. For example, reduced velocities can cause individuals of some species to increase the size of their defended “territories,” thereby displacing smaller and less aggressive fish. As aggressive fish control larger areas of stream habitat, there is less available for others.

Streambank cover is important to many species of game and forage fish in larger rivers, such as smallmouth bass, rock bass and many members of the sunfish family and others. But when areas near the streambanks are left “high and dry” these species may be forced into less suitable habitat in the middle of the stream.

Also, reduced flow after spawning can leave fish eggs stranded out of water, decrease current velocity needed for the successful hatching of some species, or crowd nursery habitats.

Some stream-dwelling animals may be impacted as well. Heicher noted in 1995 that entrances to muskrat dens in wetlands along streams were often left exposed, making them vulnerable to predators.

Low flows could also post problems for waterfowl, which often land, rest or feed along the rivers. Low flows allow predators to gain access to islands and wetland areas that would normally be isolated and protected.

Problems may not end with the drought, Heicher said. “After the rain starts to fall and the water’s back in the streams, people begin to forget about it very quickly,” he said, “but the biological recovery of that stream may take longer.”

For example, he said, while fish from a large, warm water river may recolonize a small warm water tributary as soon as the flows increase, it may take longer for insects and bottom-dwelling organisms to recover.

Recovery may take even longer for a cold water stream that flows into a warm water river: It may be cut off from the insect and fish populations it needs for recolonization.

“It would depend on a stream-by-stream basis what kind of an effect you would have,” Heicher said.

As dry spells become more frequent, some species that are slow to recolonize could be disproportionately hard hit, gradually changing the biological makeup of the stream. “I’m not saying that 10-year droughts are OK and five-year droughts are bad,” he said, “but obviously if you’re going to have these stresses on a more frequent basis, it’s going to have a biological effect.”

Copies of Heicher’s report, “Effects of Low Flow on Environmental and Recreational Resources of Major Rivers in the Susquehanna River Basin,” are available from the SRBC by calling 717-238-0423.