HYDROLOGY OF THE ULAO
By Dale Buser - Environmental Consultant
In the Midwest, the word “hydrology” is often incorrectly
used to refer to surface water alone. Hydrology is the study of all
waters. This includes surface-water bodies such as wetlands, creeks and ponds;
ground water such as that pumped from wells or discharging from seeps and
springs; water falling as rain or snow, evaporating from the land, and used by
plants. The hydrologic cycle is complex. Consequently, solving problems dealing
with water commonly necessitates study of a wide variety of factors, large
areas, and long periods of time.
Where does Ulao Creek’s Water Come From?
Simple question … not so simple answer. Even though
intermittent creeks flow into the Ulao Swamp, Ulao Creek is usually defined as a
perennial stream flowing out of the south end of the Ulao Swamp. Perennial
streams continue to flow year round, but can occasionally go dry during periods
of extreme drought. In contrast, intermittent streams are normally dry, flowing
only in response to intense precipitation and snowmelt.
The portion of the creek’s flow that is not derived from
immediate surface runoff is called “baseflow.” Ulao Creek’s baseflow is derived
primarily from springs and seeps in the western half of the Ulao Swamp. The
Creek’s baseflow is modest (e.g., 100 gallons per minute or less in the summer),
but is critical to aquatic life. Without this sustained source of water, the
creek dries out and aquatic organisms perish, migrate, or become dormant.
Baseflow is not a concern in regard to flooding problems.
Flooding in the watershed is caused by surface-water runoff.
Clayey soils and increasingly urban land uses cause storm and melt water to
quickly flush to streams. This water is usually heavily laden with sediment and
nutrients and can damage the creek’s biota and morphology. Storm-water
management practices such as detention ponds are designed to help buffer the
creek from these intense surges of water, nutrients, and sediment.
What Does Geology Have to Do with Ulao Creek and Ulao Swamp?
Sediments underlying the watershed are composed of a
complexly interlayered sequence of sand, silt, clay, and some gravel. The
thickness and extent of these layers dramatically affects the way precipitation
infiltrates into the ground or runs off the land, and the direction and velocity
of ground-water flow. To summarize in a very small nutshell, most of the Ulao
Creek watershed is underlain by fairly impermeable clay, a sediment impeding
infiltration and ground-water discharge. Modest-sized areas are underlain by
permeable sand and gravel, sediment conducive to surface-water infiltration and
ground-water discharge. These areas are important to allow stormwater to
infiltrate, attenuating flood flows. This same water raises water tables and
helps provide clean, cool baseflow to the creek in times of drought. Those same
conditions are favorable to springs and seeps, the features that sustain Ulao
Creek’s flow during dry weather.
Study Group Formed
Earlier this year, a group of landowners from the watershed
banded together with scientists and local government to form the Hydrology Study
Group. The Hydrology Study Group has met
several times over the late winter, spring, and early summer
to examine data already collected, identify issues important to the vitality of
Ulao Creek and Ulao Swamp, and set goals to protect, and, if possible, improve
the creek and swamp. Some of the issues identified by the group include
Higher than normal water levels in the Ulao Swamp.
Vegetative change in the Ulao Swamp from a lowland forest
Low to no baseflow in the creek during drought
Invasive species such as reed canary grass and hybrid
In response to these concerns, the Hydrology Study Group set
three goals to help maintain/restore Ulao Creek to a more healthy perennial
stream – a stream suitable for providing nursery areas for migratory cool water
game fish and other important organisms.
The goals are:
Prevent further decline of the forested wetland community
in the Ulao Swamp
Control the spread of invasive species, with particular
emphasis on the Ulao Swamp and the corridor immediately adjacent to Ulao Creek
Stabilizing or increasing dry weather baseflow for the
benefit of aquatic and riparian plants and animals.
Although work on these goals has barely begun, the group has
identified one concern that may help promote these goals. The Ulao Swamp was
historically a forested wetland, an area occupied by water loving trees such as
silver maple, elm, and green ash. While the swamp could be flooded for weeks on
end, it usually dried out during summer. Since the 1970s, year-round flooding
has become increasing common in the swamp. These conditions have caused much of
the forest to die (take a look from Ulao Parkway some time).
The group discussed what changes occurred to cause this to
happen and toured the creek upstream of Ulao Road. Two significant disruptive
events occurred during the late 1960s and early to mid 1970s: the die-off of
large tracts of American elm due to Dutch elm disease, and extensive damage to
trees caused by a severe ice storm. Both of these events contributed
inordinately heavy loads of woody debris to the forest floor. Much of this
debris fell directly into the creek channel or floated to the channel during
higher water periods. The large tangle of fallen wood created log and brush jams
that, in turn, caught sediment and other debris. The large amount of woody
debris was very evident during the tour of the creek. The natural “dam” created
by the wood, debris, and silt caused water levels in the swamp to rise. Rising
water levels killed more trees, causing another heavy load of woody debris to
accumulate and thus causing water levels to rise even further. This cycle
appears to be continuing with the decline of trees in ever larger areas of the
The Hydrology Study Group studied the morphology of the creek
and found that large volumes of soft sediment were trapped by piles of partially
buried woody debris in many areas. A long-time resident of the area stated that
the creek was much deeper in the late 1960s.
With this in mind, the group believes that the excessive
woody debris deposited in the creek beginning during the 1970s caused the stream
channel to aggrade, raising the elevation of the swamp’s discharge point, in
turn raising water levels in the swamp. If the excessive woody debris would be
cleared from the creek channel, natural processes would, over time, return the
stream bed to a more natural lower elevation. This in turn would lower dry
weather water elevations in the swamp and would increase dry weather flow in
Ulao Creek. Lower water levels also would allow lowland hardwoods to survive and
recolonize areas presently flooded for most of the year.
Members of the group are conducting a small scale experiment
on this approach in a debris choked area just downstream of Ulao Swamp.
Excessive woody debris that formerly lay across the channel was moved by hand to
lay parallel with the stream banks. Stream bottom cross sections were measured
and will be re-measured at a later date to determine if natural processes have
caused the channel to deepen. We’ll tell you more in a future issue of the