Total precipitation at Tahoe City for the water year (October 1 - September 30) is projected to be 26 in for 2012.
Mean annual precipitation is 31 in. The middle quartile values are from 21 - 43 in.

Flow is controlled by the dam at the Lake Tahoe Outlet in Tahoe City.
Flow is adjusted based on several factors.
Minimal water supply for Reno
Controlling Lake Tahoe Level.
2016-2017 Season - Record Snowfall After years of drought the level was at the rim 6,223.
The maximum level so piers are usable and lakefront property is not flooded is 6,229.1'
In the middle of May, 2017 the lake was up to 6,227.7', so there was only 1.4' to the maximum level.
Projected rise from snow melt thru early August was projected to be 2.7', so water was being released at the maximum rate to avoid overflowing bridges and other problems.
See
High water on the Truckee River Explained | Video from Placer County Emergency Manager
USGS Current Conditions for USGS 10337500 TRUCKEE R A TAHOE CITY CA

Table 3-1. Selected Upper Truckee River Basin Truckee River Inflows Above the Farad Gaging Station
(Average Annual Runoff Volumes in Acre-Feet [Flow Rates in Cubic Feet per Second])

Gaging Station Location	Average-Water Year (af [cfs])	Low-Water Year (af [cfs])	High-Water Year (af [cfs])
Lake Tahoe Outlet (at Tahoe City)	161,450 [223 cfs]	110 [0.15 cfs]	832,570 [1,150 cfs]
Donner Creek (at SR 89)	60,890 [84.1 cfs]	18,750 [25.9 cfs]	102,800 [142 cfs]
Martis Creek (below Martis Dam)	19,470 [26.9 cfs]	5,000 [6.90 cfs]	53,940 [74.5 cfs]
Prosser Creek (below Prosser Dam)	65,950 [91.1 cfs]	17,660 [24.4 cfs]	154,930 [214 cfs]
Little Truckee River (below Boca Dam)	123,800 [171 cfs]	40,250 [55.6 cfs]	340,270 [470 cfs]
Bronco Creek (at Floriston, California)	9,920 [13.7 cfs]	4,400 [6.06 cfs]	15,420 [21.3 cfs]

af = acre-feet cfs = cubic feet per second USGS = U.S. Geological Survey
Source: Farad Diversion Dam Replacement Project - Chapter 3 - Hydrology, 2002

Mean annual precipitation in the Tahoe basin is 31 in. The middle quartile values are from 21 - 43 in/year.
Assuming runoff is proportional to precipitation this means that about 25% of the time all the water entering the lake would be lost to evaporation, so output to the Truckee River is restricted and the lake level will be lower at the end of the year.
Outflow to the Truckee River is adjusted to keep the water level within these limits. However, there are minimum flows to supply water for Reno and maximums to avoid flooding in Reno.
See the Floristion Rule

According to the Truckee River Chronology at nv.gov, Oct, 2011, Truckee river outflow can range from 110 acre-ft [0.15 cubic ft per sec (cfs)] to 832,570 acre ft [1,150 cfs].
The minimum according to another source is 70 cfs.

Mean monthly flows (cfs) in the Truckee River at Donner Creek based on model results in wet, median, dry, and extremely dry hydrologic conditions. Shaded boxes indicate when recommended minimum flows of 75 cfs are not met.
Source: Truckee River Riparian Vegetation and Fluvial Geomorphology Study, 1993

	Wet							Median
	Apr	May	Jun	Jul	Aug	Sep	Oct	Apr	May	Jun	Jul	Aug	Sep	Oct
CC	1577	1552	1256	547	442	455	340	623	778	518	173	110	223	260
NA	1504	1516	1252	543	441	458	347	602	762	517	174	112	226	270
LVNA	1504	1516	1252	543	442	458	348	603	763	516	174	112	228	271
TROA	1534	1546	1293	544	329	307	309	621	805	551	215	118	137	202
	Dry							Extremely Dry
	Apr	May	Jun	Jul	Aug	Sep	Oct	Apr	May	Jun	Jul	Aua	Sep	Oct
CC	275	289	158	90	68	89	23	227	253	133	63	26	44	5
NA	273	282	158	89	67	73	29	229	253	130	60	24	34	12
LWSA	273	282	158	89	66	72	31	229	253	130	81	24	32	14
TROA	272	391	247	105	68	61	41	201	260	142	64	22	35	21

 CC - Current Condition
 NA - No Action
TROA - Truckee River Operating Agreement
LWSA - Local Water Supply Alternative

Source: Truckee River Operating Agreement usbr.gov, 2008

Table 1. Statistics for major reservoirs in the Truckee River Basin.

Reservoir Name	Storage (Acre-Feet)	Drainage Area (mi2)	Date dammed	Elevation	Function(s)
Lake Tahoe	744,6001	506	19132	6227	Storage for instream flow and irrigation
Donner Lake	9,500	14	1930s3	5900	Municipal and irrigation water supply
Martis Creek	20,300	40	1971	5760	Flood control
Prosser Creek	29,800	50	1962	5680	Flood control, instream flow, water conservation in Lake Tahoe
Independence Lake	17,500	8	19394	6300	Municipai supply
Stampede Reservoir	226,500	136	1970	5760	Pyramid Lake fish flows and flood control
Boca Reservoir	41,100	172	1937	5600	Municipal and irrigation water supply

1. Only the top 6 ft of Lake Tahoe that is controlled by the dam is counted.
2. The first dam was constructed in 1871 or 1874
3. Original dam was constructed in 1877
4. Small dam originally constructed in 1879
Source: Truckee River Riparian Vegetation and Fluvial Geomorphology Study, 1993

Links:
Truckee River Operating Agreement TROA.net
Truckee River Operating Agreement usbr.gov, U.S. Bureau of Reclamation, 2008
Truckee River Operating Agreement (Biological Resources), U.S. Bureau of Reclamation, January, 2008
  Includes "Truckee River Riparian Vegetation and Fluvial Geomorphology Study", U.S. Fish and Wildlife Service, Sept., 1993
  Executive Summary

Truckee River Chronology at nv.gov, Oct, 2011
California Creeks - Truckee Floriston
www.truckeeriverwc.org/about/watershed