Table 7. Gross sedimentation rates in 1992 and 1993.
Range (mm)
Morphological
Summer
Winter
Summer
unit
92
9293
93
Levee
12
1 5
15
Vegetated upper mudflat
36
3 10
18
Unvegetated lower mudflat 5 12
7 12
3 10*
Pond--plate
18 35
8 28
2 12
Pond--cup
5 38
6 15
Marsh
2 10 (est)
ND†
ND
Gully
100 300
28 75**
60**
* Larger values from craters.
† ND = no data.
** Based on small sample size.
Table 8. Number of flooding events reaching
critical heights to cover landforms in 1992 and
1993.
Number of flooding events
Critical height
Summer 92 Winter 9293 Summer 93
Ponds (4.6 m)
21
66
23
Mudflats (4.87 m)
13
36
15
Levees (5.21 m)
2
9
4
events covering each landform. During the en-
dated, but other factors appear important as
tire period, 110 tides exceeded the 4.6-m ele-
well. Assuming a general elevation of 4.6 m for
vation necessary to enter the ponds (Table 8). In
ponds to flood (a 9.48-m Anchorage datum tide),
contrast, levee elevation data suggest that dur-
Table 8 shows a minimum of 21 sediment-bear-
ing the same period, only 15 tides were suffi-
ing tidal floodings would have filled ponds,
ciently high to inundate them completely. The
mainly during the late July to early August and
average sediment accumulation in the ponds
late August to early September tidal cycles.
from May 1992 to September 1993 was 46.5 mm,
Through September 1992 to May 1993, the
yielding a gross rate per tidal cycle of 0.4 mm.
ponds were inundated by a minimum of 66 sedi-
The average rate per tidal flooding on levees is
ment-bearing tides monthly (except December),
0.3 mm.
with the highest tides in March and April. Al-
The higher rates of sedimentation in ponds
though there may have been additional flooding
than on river levees clearly show the more im-
during summer because of higher discharges,
portant role of tidal inundation than Eagle River
the range in sedimentation rates for ponds dur-
floods as a sediment source and thus, within the
ing the winter with more flooding tides was
study area, sedimentation is tidally dominated.
similar to that of the previous summer (1 to 38
The large amount of suspended sediment in
mm). This similarity may reflect several other
Knik Arm increases its importance relative to
factors, including lower sediment concentrations
the Eagle River, and contrasts with other tidally
in winter flood waters, or ice cover effects, such
dominated salt marshes and mudflats where
as the freeze-on and removal of pond bottom
glaciers are not important sources of marine sus-
sediments during freezeup or breakup. From
pended sediments. The role of the river is less-
May to September 1993, 23 sediment-bearing
ened by its apparently lower suspended sedi-
floods occurred, mainly from July through Sep-
ment transport rate, including the lack of sedi-
tember. These rates were generally lower than in
ment influx during winter.
either the summer or winter of 1992, ranging
The amount of material that originated by re-
from 1 to 15 mm. The reasons for these reduced
suspension of pond bottom sediments ranged
rates are not yet known.
from 8 to 16 mm for the summer of 1992, or ap-
Gross sedimentation rates were higher within
proximately equivalent to the thickness of new
ponds than on river levees (Table 7); this ap-
sediments deposited in the ponds during the
pears related to the total number of flooding
33
Previous Page
