Watershed Field Science Exercise 1
Monday, May 11, 2009
Watershed intro, geology, sediments, habitat, and groundwater discharge
Welcome to the Melosira! Today we will be re-introducing you to thinking about
watersheds, especially the Winooski River watershed, and the many facets that affect
water quality and habitat ecology for systems worldwide. We will take a look at landuse
in watersheds from the unique perspective of the lake, and start thinking about sediments
and water flow in watersheds, focusing on the role of the geologic setting for sediment
sources and groundwater flow. We will be investigating several sites around Burlington
Bay this morning, visiting the ‘General’ discharge and sampling sediments in the harbor
and surroundings to start thinking about groundwater discharge and sediment transport
from the watershed to the bay before moving on land to look more closely at a large fault
system exposed at Lone Rock Point and investigating sediment composition at North
Beach.
Exercise 1: Introduction to the watershed – using the attached topo map and your view of
the Burlington area, sketch in the general land use categories represented – principally the
difference between urban areas (concrete pavement/buildings) and green space
(grass/trees).
Thinking about water flow and the things water carries with it into the Lake, in what
ways are these urban spaces different from green spaces?
Exercise 2: We will now head over to ‘The General’, a large pockmark about 100 feet
deep in Burlington Bay and about 100 feet in diameter. Take a close look at the sonar
images as we pass over this to ‘see’ the topographic expression of this groundwater
discharge feature – why is it shaped this way?
We will also take some sediment samples within and outside The General, describe the
difference between these 2 samples:
Exercise 3: Champlain Fault – we will travel now to Lone Rock Point, a famous example
of a thrust fault. The rock above is older than the rock below the fault. Sketch the fault
below and describe the direction of movement (we will take a much closer look this
afternoon!)
Exercise 4: We’ll now take a look at some of the sediments in Burlington Bay near the
breakwater, not too far from the General Butler shipwreck. Check out the sonar scans as
we pass over the wreck site. After we get a sample of the sediment, describe what you
see (by eye and using the microscope on board):
Exercise 5: Looking again at the sediments we collected at the 2 generals, what sort of
organisms do you see here?
How does the surrounding geology affect the habitat potentially available for organisms
in these sediments?
How does human use of watershed and waterfront affect the habitat for these organisms?
Part II (On Land). A key part of this afternoon’s work will be to start thinking about how
the geology of the surroundings (the composition of the rocks and sediments, faults and
fractures that affect groundwater flow) are part of the watershed system. You have 2
maps as part of this – a bedrock geologic map describing the underlying (or
exposed/outcropping) rocks, and a surficial geology map describing the sediments.
Exercise 6: Having looked at ‘the General’ feature on the Melosira we will now take a
closer look at the Champlain thrust at Lone Rock Point to see if they are connected. At
Rock Point: Using your compass, try to measure the direction of the fault plane exposed
at different parts of the outcrop. Also collect a sample of both the Iberville/Stony Point
shale and the Dunham dolomite from the rock fall at the base of the outcrop, and collect a
sample of sediment.
Using this measurement of the fault plane, extend a straight line on your map across the
bay. Does it interest the position of the General?
You might note that the extension of this fault on your map (Map 1) does go into the bay,
and that it is curved, in places missing, and in other places displaced by other faults.
Now think about how your extension of the line as a straight feature would be affected by
a change in topography (or bathymetry when we consider the lake depth) – does this shift
the line to the east or the west?
Exercise 7: Take a close look at the material within the fault plane of the Champlain
Thrust and compare it to the same rocks just outside the fault plane. Describe what you
see:
Measure (best you can - it is an estimate) the density of fractures in this material (# per
square inch). Water will flow in fractures – why do we not see much water flowing from
these fractures if the General pockmark is such a big feature?
Exercise 8: Materials. We’ll take a look at the rocks and minerals making up the material
at Rock Point, North Beach sand, and some sediments collected in Stowe using hand
lenses. You will use these small magnifiers to see some of the different minerals and
rock fragments that make up each to get used to establishing differences related to the
origin of a sediment. From the sand and soil samples, look for:
Quartz – clear/white, irregular shape
Feldspar – white to pink, rectangular
Mica – clear to black, flaky and shiny
Chlorite – green, irregular shape
Describe the minerals and rock fragments you see along with something new (feel free to
ask for help from Greg, Lydia, or other students!!):
Let’s look at the surficial geology map (Map 2) and think about the sediment that is part
of the immediate surroundings of this watershed system and where it may have come
from. Describe 4 different ways in which these surficial sediments could have been
deposited:
Map 1 – Bedrock Geology map of the Burlington Area. Each color and 3-letter
designation denotes a different geological unit (where first letter indicates the geological
period); Orange Cdh is the Cambrian Dunham dolomite, light purple Osp is the
Ordivician Stony Point shale; green Oib is the Ordivician Iberville shale; tan Cm is the
Cambrian Monkton quartzite. The light red in Burlington denotes an urban area
(concrete…) while the white, essentially blank stuff is alluvial cover (see the next map).
Map 2 – Surficial Geology map of the Burlington Area. Each color and letter designation
denotes a different sediment unit. Purple is artificial fill, light blue (mfs) is medium fine
sand; turquoise is river deposited alluvium (al); light grey is glacial till (t); brown is
medium coarse sand (mcs), and red is Champlain Sea clay (s-c).