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Falls Lake is the sole drinking water
source for the 350,000 people in
Raleigh, Wake Forest, Rolesville,
Zebulon, Knightdale and Wendell, and
this spring and summer it is dropping
between three and five hundredths of a
foot every day.
On Monday it was a foot
lower than its normal level of 251.5
feet above mean sea level.
The U.S. Geologic Survey,
which has been monitoring stream flows
for over 100 years, issued a warning
statement at the end of May. “Despite
some rainfall at the end of the month,
stream flows during May in North
Carolina were at or near record low
levels, particularly in the western part
of the state.”
The USGS also said
groundwater levels are falling across
the state though the amount of decline
varies according to the underlying rock.
Finally, “Normally
the lowest stream flows of the year
occur in late summer, when water use
demands are highest, and in the fall. If
below average rainfall continues through
the summer and fall, new record low
flows are likely to occur in many of
North Carolina's rivers.”
The only way to replenish
Falls Lake is for the area – and
particularly the Falls Lake watershed
above the dam – to have significant
rainfall. Aside from the possibility of
a hurricane striking the North Carolina
coast and moving inland, there is no
sign of significant rain for the area.
The short-range forecast is for
scattered showers with an increased
chance of rain over the weekend.
When rain does fall in the
Falls Lake watershed above the dam,
there are two factors which will affect
the lake’s replenishment: the size of
the watershed and groundwater.
Falls Lake is a relatively
small impoundment with a small watershed
even though the Neuse River is a
significant North Carolina river as it
descends into the Pamlico Sound.
Falls covers 12,410 acres in
Wake, Durham and Granville counties and
has a watershed of 770 square miles.
Compare Falls to Jordan Lake
nearby, which covers 13,940 acres – just
slightly larger – but has a watershed
above the dam of 1,690 square miles –
twice the size of Falls’ watershed.
In that 770-square-mile
watershed, there are two significant
factors, the soil and development.
About a fifth of the
watershed is in one of the East Coast’s
Triassic Basins, geologic areas formed
during the Triassic period of the
Mesozoic Era. That was a time when the
great tectonic plates were pulling apart
to eventually form our present
continents. The strain created rift
valley which then were filled with muddy
sediments which formed rock. As the rock
was eroded and decayed, the soils
created were dense and clayey, making
great bricks and tiles.
“The soils in the Triassic
Basin are recognized as being very
impermeable,” Curtis Weaver with the
Raleigh USGS office said this week.
“Water that falls onto Triassic Basin
topsoil is going to run off and the
infiltration rate is very low. You’ve
got to have infiltration into the
groundwater” to have a base flow.
Weaver said there are two
contributors to the water in a stream,
the runoff from the adjacent land “and
the base flow that comes from the
adjacent water table. The source is the
ground water.
“It is not uncommon to have
a stream with a large drainage basin to
have a low flow or no flow,” Weaver
said.
Falls Lake is fed by three
rivers. The Little River merges with the
Eno River, which then quickly joins the
Flat River to create the Neuse River.
The USGS has been measuring
the flow in the Flat River at Bahama
since July of 1925. Weaver looked at the
flow Monday afternoon and said it was at
21 cubic feet per second “and the median
flow is twenty-five to thirty cubic feet
per second.” Last week on Wednesday and
Thursday the flow was 15 cfs. “It’s
definitely below the median.”
The same was true for the
Little River near Orange Factory. “As of
this afternoon its at nine cubic feet
per second, yesterday it was at four to
five compared to ten to 15 normally for
this time of year,” Weaver said.
As you move out of Durham
County into the Wake County part of the
Falls Lake watershed, the problem is not
the soil but the increasing amount of
impermeable surface created by the large
homes, driveways, roads and structures.
Wake County has restricted
development in the Falls Lake watershed
by banning municipal water and sewer and
requiring large residential lots, about
one acre throughout the basin, two acres
nearer the lake.
But several individuals and
groups in recent years have begun to
point out the growing impermeable
surface area and question its affect on
groundwater recharge and the lake’s
future.
Finally, hot days increase
the amount of evaporation from the lake
surface.
Weaver ended the interview
with a caution: “We have been abnormally
dry for a significant period. There is
definitely room for concern and everyone
should be aware of the need to conserve
water.” |
